Port forward issue

I have some port forwarding shenanigans going on, I enabled DMZ on my modem to my GL-A1300 router, enabled port forwarding on 1111 pointing to my server which has a service running on that port. DDNS service points to my modem that then does all that routing and finally arrives to the server. Everything works fine, if I got with 5g to the DDNS:1111 it connects perfectly, from my GL subnet it works too, but not from my Modem subnet
What’s the problem here? If the service works from outside my network and inside, it should also work from the modem one. But somehow it can’t resolve the DDNS from modem subnet.

I would say it’s an routing issue. Maybe your modem does not know about the correct routes?

I tried traceroute from the 2 different networks and it’s basically the same, GL one just does 1 more hope to _gateway at first and then public ip connected to DDNS.

Could it be that as the ddns points to the Modem, it just does an infinite loop of some as it’s trying to connected to himself? That wouldn’t make sense because GL can connect fine.

But this makes me question another problem I have, my GL ip on Modem is, if I try while on Modem wifi it doesn’t work, on GL it is able to connect just fine again.

Could you help me admon or anyone else too? Thanks