I once bought a router to use for my internet when I moved into my new house just to find out that it “wasn’t compatible” with Verizon’s service. I still have it (because I’m terrible about returning things). Is there any point in keeping it? Is there anything fun or interesting that I could do with it?
The easiest way would be to just bridge the ISP router if it supports that feature. To connect back to the ISP router’s webui you can setup outbound NAT on your second router (generally only available on enterprise gear). Don’t double NAT unless there’s no other option.
If you can bridge, the sky’s the limit in terms of what gear you can have on the backend. My bridged modem is directly connected to a L3 switch which links to my Proxmox cluster, and my Opnsense router exists virtually in the cluster and can be migrated freely between the machines.