I kinda had a similar problem. Never found the root cause, but what did the trick for me was to put an OpenWRT Router between the default ISP router and my home network.

As I said, I never figured out, why Android did not respect the DHCP settings of the default router, but here we are. Maybe it was some DNS shenanigans by the ISP’s config, maybe it was a wrong DNS/DHCP configs from my side, maybe it was IPv6 shenanigans. Those are the culprits I would investigate from your side.