I just checked my router control panel and the DHCPv6 server is already off so the problem might not be in my router but in Android 15. In order to change the default DNS addresses in my router I need to reset it to factory default and although it's pretty easy and I only need a bunch of parameters to sign-in to the VoIP service I would much rather limit the custom DNS values to the devices I use and let everyone else use the default ISP DNS addresses
I also found this one thread where you also helped someone with a similar problem and tried setting my router the same way he did: IPv6 DNS configuration on ZTE H6645P - #7 by Deltwin
but instead of using a real IPv6 I entered a dummy address: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Whilst the setting is turned off, this will also hopefully act as a contingency plan in case it turns itself back on. In theory, a non-existing address should simply not work meaning the IPv6 DNS resolution should fail and fallback to my primary custom choice