Thanks again for all your help - turns out I can't change the server address range on the router the ISP gave me because its a basic ONT + wifi point. All working except for accessing the GUI. Have a great day!
You should be able to access the original GUI if you put your laptop onto a temporary static address in the same network range and subnet mask as the original router. Of course, you won’t be able to access the rest of your network until you put it back on a dynamic address again. Effectively, you’ve put them into two separate logical networks, even if they are all in the one physical network.