I had a router from BT that despite having won awards, could not resolve its external IPv4 address internally. It knew what its external address was and knew that it had to DMZ everything to an internal address, but if you attempted to resolve the external address (expecting it to go to your DMZ'd machine), it'd fail.

I also had a router from another UK ISP that would crash and hang if you sent it a malformed IP packet (ie put its respond-to address as itself); it'd go mental replying to itself and then processing the messages it sent to itself. Not quality.

The EE broadband router I had would hand off its internal web admin session to external visitors if they connected whilst a web admin session was in progress. Yes, let's hand our internal web admin session page to an external visitor....

BT sent another hub that looked identical and worked correctly; underneath it was a Siemens instead of a Thompson I think.

So all in all, I have found that the ISP boxes were never really finished, and some (like Sky's box) needed rebooting periodically as I recall, so buggy too.