My ISP won't give me the router credentials because they claim that that would somehow risk their network interconnectivity or something. I have no idea how this makes any sense since a router (shouldn't) give me that level of access, but maybe I'm missing something?
If the router is also the modem, it can have diagnostic stuff that could (I am careful to point out: theoretically) cause them a big support headache in one way or another.
Also, I had a DSL modem back in the day with its own terrible NAT built-in. I managed to log in (I may have been given the admin password from a friend who worked at the ISP, allegedly) and once I put it into bridge mode, I was able to acquire multiple public IPv4 addresses via DHCP.