Starting from a factory reset:

• I plugged in an LTE modem.
• Interface lte1 appeared.
• A dynamic entry appeared under IP > DHCP Client that gets an IPv4 address.
• Under IPv6 > Settings I disabled Disable IPv6 and set Accept Router Advertisements to yes.
• Under Interfaces > LTE > LTE APNs > default I set IP Type to IPv6.
• Under Interfaces > LTE > LTE APNs > default I also set IPv6 Interface to lte1.
• Under IPv6 > DHCP Client I added an entry with interface lte1 and address and prefix checked, which stays in "searching..." mode.
• I only have a link-local address on the lte1 interface.

Anything else I need to do to get an IPv6 address?

Your carrier may need a specific APN to get IPv6. Try googling for "APN <carrier_name> IPv6" or similar. Or perhaps calling customer service for your carrier to ask them if they support IPv6 and what APN to use.

On Mikrotik, uncheck the "Use Network APN", and use the APN you found from Google/etc in the APN Profile's apn= .

Ok, I found out what was the issue. Two issues actually.

First issue was that the "modem" I'm using seems to actually be an Android device and because the web UI is pretty horrible I haven't found out how to enable the IPv6 protocol.

So I plugged in an actual Android phone with IPv6 enabled and I successfully got an IPv6 address.

But if I go to Interfaces > LTE > LTE APNs > default, change something (it doesn't really matter what I change, can be "Use Peer DNS") and click "Apply" the IP address gone and won't come back until I unplug and re-plug the phone. Seems a lot like a RouterOS bug to me.

Starting from a factory reset:

• I plugged in an LTE modem.
• Interface lte1 appeared.
• A dynamic entry appeared under IP > DHCP Client that gets an IPv4 address.
• Under IPv6 > Settings I disabled Disable IPv6 and set Accept Router Advertisements to yes.
• Under Interfaces > LTE > LTE APNs > default I set IP Type to IPv6.
• Under Interfaces > LTE > LTE APNs > default I also set IPv6 Interface to lte1.
• Under IPv6 > DHCP Client I added an entry with interface lte1 and address and prefix checked, which stays in "searching..." mode.
• I only have a link-local address on the lte1 interface.

Anything else I need to do to get an IPv6 address?

I recently went through this exact situation and found a simple and effective way to solve it.

1. Remove any manual configurations you added under IPv6 > DHCP Client, as well as any manually created prefix pools or static IPv6 addresses.
2. Go to Interfaces > LTE APNs > [YOUR_APN_NAME] and set the IP Type field to Auto. This allows the modem to negotiate both IPv4 and IPv6 automatically with your mobile operator, avoiding conflicts or limitations.
3. Very important: just below, in the same APN menu, there is an IPv6 Interface option. Set this to bridge (or whichever interface connects your internal network devices). The most common mistake is to leave this set as lte1, which causes the valid IPv6 address to be bound only to the cellular interface without being routed to your LAN.
4. Under IPv6 > Settings, make sure that Disable IPv6 is unchecked (i.e., IPv6 is enabled), and that Accept Router Advertisements is set to yes.

After these settings, there is no need to manually add a DHCPv6 client or configure any prefix pools or static addresses. The MikroTik router will automatically create a default route and assign the global IPv6 prefix to the bridge interface, allowing your internal devices to receive valid IPv6 addresses.

To confirm everything is working correctly, go to IPv6 > Addresses and check if a global address (usually starting with 2804: or similar) is assigned to the bridge interface. Your internal devices should also begin to receive IPv6 addresses automatically.