Hi everyone I'm posting this guide here in hopes it may help someone. I saw a few topics talking about this but those steps alone didn't help me. I'm posting the configuration that finally worked for me. For full transparency I am using the Rooter firmware based on OpenWRT so it's possible that something doesn't translate but I think it should be similar. The Rooter thread is really hard to search and follow so posting here in hopes someone can find this.
I have an Archer C7 v2 router and an inseego m2000 5g modem plugged in via USB. For a long time I was using this setup unaware that ipv6 was NOT working when connected to the router. When connected to the m2000's own wifi Ipv6 worked so I knew it was a configuration issue.
Under Network -> Interfaces:
I had an odd issue where the device for wan6 was incorrect so make sure it's the correct device first and that you can see it getting IPs at all.
Then change your LAN device with these settings: Ra service, dhcpv6service, NDP proxy all relay mode, don’t ignore interface, delegate prefixes set.
Wan6: same settings as LAN device but very important check box designated master.
I'm not 100% sure if this next step is needed but this is what I ended up with, ssh into the router and edit /etc/config/network (I like using nano for this over vi). Under wan6 add the setting option ip6prefix 'aa:bb:cc:dd::/64'
with the IP prefix taken from the top Ipv6 address that WAN6 was able to get, should be a longer one than the internal Ipv6.
At this point Ipv6 should be resolving. But in my case I wasn't done. I was having random drops and cut outs that were unbearable. To fix this I adjusted the MTU. I experimented to see what the highest MTU I could put on wan6 was and fix the slowdowns, in my case it was 1480. I added the mtu under wan6 in /etc/config/network like so:
option mtu '1480'
One thing to be aware of when choosing the MTU is that if you put it too low only mobile devices would connect and my desktop would refuse to connect. So I would experiment and try to test the highest MTU you can get without issues. For me I reduced it slowly to 1480, at each level it seemed to be slightly better but only at 1480 did the random drop outs stop.
Now this part I'm not 100% sure about. You may want to experiment with different MTUs on the LAN and WAN device. I tried setting the LAN to 1500 (option mtu '1500') but now I have set everything to 1480 including WAN. I don't totally understand this part so let me know.
My final step was to go into firewall, edit LAN and chec MSS Clamping. Without MSS Clamping checked there seemed to be problems even when I set all the MTUs to be the same number, but maybe it's not needed.
After all this the device and network seems to be a lot more stable even fixing some issues I was having with ipv4 only somehow. For example my steam downloads would crash to zero and then pop back up. Those drops would recover unlike the ipv6 drops which would completely cut out and terminate pages. It's possible I should have adjusted all the MTUs to 1480 due to this but it seems to be working for now.
I don't really know what I'm doing so anyone who can offer fixes and explain anything is very welcome!