Hello,
If the RUTXR1 was configured with static IPv6 entry, they most likely do not match the current LAN IP. You’ll need to set a static IP (either in IPv4 or IPv6) in the same range as the previous modem was configured in, then you should be able to reach the WebUI of the router. Set the LAN zone to DHCP (or change to a different static entry within the new network), and remove the static IP from your PC. On the ISP provided modem, you should see a DHCP lease for the RUTXR1, and should be able to reach the WebUI using this address.
Best regards,