Suggestion 1:
If you've got an alternative browser available on your iPhone, try accessing the router's settings with it. (That definitely applies if you're currently using Chrome, as it's well-known for generating that message; Safari is possibly less likely to but, if that's what you're already using, try something else anyway).
Suggestion 2:
Leave your iPhone out of things. Temporarily connect a desktop PC or laptop to the router (ideally with an Ethernet cable but a wifi connection would probably work just as well for this task) and then try using it to get into the router settings. (Avoid using Chrome. Use Internet Explorer, Edge or Firefox).