I've done something similar in Ireland, where ISP router was the only way to connect. Managed to setup everything on OpenWRT router, but it kept disconnecting, so I put openwrt router behind ISP router.
Interesting thing I found in ISP router is DMZ host - just point it to your own router and that's it. Basically ISP router doesn't exist lol.
Then you have absolute freedom with your router.
Some time ago I've done a "public IP implementation" on my VPS when I was on mobile network (no public IP).
Basically set up IPSec/Wireguard on VPS and connect your router to it. Then setup EoIP over VPN between VPS and your router. Then add EoIP tunnel to your LAN's bridge in your router.
Then setup all ports forwarding (using iptables) from your VPS to your router on LAN, so if you connect to your VPS using tcp80, it will be simply forwarded (NAT'ed) to your router. Except tcp22, for SSH to your VPS obviously...
And now you have yet another public IP lol.
This is not something you asked, but might give you some ideas.