I have an old MR6400 v4 and I've tried two different sims with it (Vodafone Idea, and Jio in India). 

For both sims, the router gets IP for ipv6 but nothing for ipv4 (the field is empty, not 0.0.0.0). If I use "ipv4 and ipv6" for PDP type it shows connected and it receives IP only for ipv6 which works for connecting to ipv6 sites (if I set the ipv6 DNS in my end-device, since I can't set it in the router). If I choose ipv4 only as PDP type, it stays disconnected. 

Since this is happening with two different carriers, I'm wondering if this is a known issue with the router itself? It's also possible that carriers are using similar setups and they don't support using sim from a router. Any ideas if a firmware upgrade might help or anything I could try?

Btw, I did look at the ipv6 tunnel feature but I think it's for tunneling ipv6 through ipv4? I could possibly use DNS64 and NAT64 but I'd rather not have to do that.