diff --git a/src/vector/mobile_guide/index.html b/src/vector/mobile_guide/index.html
index b2b14df38..dc14c979b 100644
--- a/src/vector/mobile_guide/index.html
+++ b/src/vector/mobile_guide/index.html
@@ -4,6 +4,11 @@
 
 <style type="text/css">
 
+/* By default, hide the custom IS stuff - enabled in JS */
+#custom_is, #is_url {
+    display: none;
+}
+
 body {
     background: #c5e0f7;
     background: -moz-linear-gradient(top, #c5e0f7 0%, #ffffff 100%);
@@ -109,6 +114,14 @@ body {
     margin: 20px;
 }
 
+.mx_HomePage_errorContainer {
+    display: none; /* shown in JS if needed */
+    margin: 20px;
+    border: 1px solid red;
+    background-color: #ffb9b9;
+    padding: 5px;
+}
+
 .mx_HomePage_container h1,
 .mx_HomePage_container h2,
 .mx_HomePage_container h3,
@@ -152,6 +165,10 @@ body {
 
 <body>
 
+  <div class="mx_HomePage_errorContainer">
+    <!-- populated by JS if needed -->
+  </div>
+
   <div class="mx_HomePage_container">
       <div class="mx_HomePage_col mx_HomePage_header">
           <a href="https://riot.im">
@@ -365,7 +382,9 @@ body {
               <p>Launch the app, and enable <strong>Use custom server options (advanced)</strong>.</p>
               <p class="mx_Spacer">In the homeserver field, enter:</p>
               <p><strong id="hs_url"></strong></p>
-              <p class="mx_Spacer"><em>Note: You shouldn&apos;t need to modify the identity server field, which by default is set to https://vector.im.</em></p>
+              <p class="mx_Spacer" id="default_is"><em>Note: You shouldn&apos;t need to modify the identity server field, which by default is set to https://vector.im.</em></p>
+              <p class="mx_Spacer" id="custom_is">In the identity server field, enter:</p>
+              <p><strong id="is_url"></strong></p>
           </div>
         </div>
       </div>
diff --git a/src/vector/mobile_guide/index.js b/src/vector/mobile_guide/index.js
index 342643b47..3be589ef6 100644
--- a/src/vector/mobile_guide/index.js
+++ b/src/vector/mobile_guide/index.js
@@ -6,19 +6,102 @@ function onBackToRiotClick() {
     window.location.href = '../';
 }
 
+// NEVER pass user-controlled content to this function! Hardcoded strings only please.
+function renderConfigError(message) {
+    const contactMsg = "If this is unexpected, please contact your system administrator " +
+        "or technical support representative.";
+    message = `<h2>Error loading Riot</h2><p>${message}</p><p>${contactMsg}</p>`;
+
+    const toHide = document.getElementsByClassName("mx_HomePage_container");
+    const errorContainers = document.getElementsByClassName("mx_HomePage_errorContainer");
+
+    for (const e of toHide) {
+        // We have to clear the content because .style.display='none'; doesn't work
+        // due to an !important in the CSS.
+        e.innerHTML = '';
+    }
+    for (const e of errorContainers) {
+        e.style.display = 'block';
+        e.innerHTML = message;
+    }
+}
+
 async function initPage() {
     document.getElementById('back_to_riot_button').onclick = onBackToRiotClick;
 
-    const config = await getVectorConfig('..');
-    let hsUrl;
-    if (config && config['default_hs_url']) {
-        hsUrl = config['default_hs_url'];
+    let config = await getVectorConfig('..');
+
+    // We manually parse the config similar to how validateServerConfig works because
+    // calling that function pulls in roughly 4mb of JS we don't use.
+
+    const wkConfig = config['default_server_config']; // overwritten later under some conditions
+    const serverName = config['default_server_name'];
+    const defaultHsUrl = config['default_hs_url'];
+    const defaultIsUrl = config['default_is_url'];
+
+    const incompatibleOptions = [wkConfig, serverName, defaultHsUrl].filter(i => !!i);
+    if (incompatibleOptions.length > 1) {
+        return renderConfigError(
+            "Invalid configuration: can only specify one of default_server_config, default_server_name, " +
+            "or default_hs_url.",
+        );
     }
+    if (incompatibleOptions.length < 1) {
+        return renderConfigError("Invalid configuration: no default server specified.");
+    }
+
+    let hsUrl = '';
+    let isUrl = '';
+
+    if (wkConfig && wkConfig['m.homeserver']) {
+        hsUrl = wkConfig['m.homeserver']['base_url'];
+
+        if (wkConfig['m.identity_server']) {
+            isUrl = wkConfig['m.identity_server']['base_url'];
+        }
+    }
+
+    if (serverName) {
+        // We also do our own minimal .well-known validation to avoid pulling in the js-sdk
+        try {
+            const result = await fetch(`https://${serverName}/.well-known/matrix/client`);
+            const wkConfig = await result.json();
+            if (wkConfig && wkConfig['m.homeserver']) {
+                hsUrl = wkConfig['m.homeserver']['base_url'];
+
+                if (wkConfig['m.identity_server']) {
+                    isUrl = wkConfig['m.identity_server']['base_url'];
+                }
+            }
+        } catch (e) {
+            console.error(e);
+            return renderConfigError("Unable to fetch homeserver configuration");
+        }
+    }
+
+    if (defaultHsUrl) {
+        hsUrl = defaultHsUrl;
+        isUrl = defaultIsUrl;
+    }
+
+    if (!hsUrl) {
+        return renderConfigError("Unable to locate homeserver");
+    }
+
     if (hsUrl && !hsUrl.endsWith('/')) hsUrl += '/';
-    if (hsUrl && hsUrl !== 'https://matrix.org/') {
+    if (isUrl && !isUrl.endsWith('/')) isUrl += '/';
+
+    if (hsUrl !== 'https://matrix.org/') {
         document.getElementById('step2_container').style.display = 'block';
-        document.getElementById('hs_url').innerHTML = hsUrl;
+        document.getElementById('hs_url').innerText = hsUrl;
         document.getElementById('step_login_header').innerHTML= 'Step 3: Register or Log in';
+
+        if (isUrl && isUrl !== "https://vector.im/") {
+            document.getElementById('default_is').style.display = 'none';
+            document.getElementById('custom_is').style.display = 'block';
+            document.getElementById('is_url').style.display = 'block';
+            document.getElementById('is_url').innerText = isUrl;
+        }
     }
 }
 
