Cannot connect to my personal wifi, stuck on "Welcome to Xfinity!" page

I have a Xfinity router. Every time I try connecting to the wifi network (It's named "XFINITY-....") it sends me to

http://connectivitycheck.gstatic.com/generate_204

I have a Samsung Tablet S7 FE, and two standard laptops, and an iPhone.

It's not my router's problem. I have 2 laptops. The first one can connect to "XFINITY-...." (without needing to download any app), the second can only connect to "XFINITY" (it's stuck on the page, and there's no app to download). The iPhone can only connect to "XFINITY" (the app does nothing). The Samsung Tablet can only connect to "xfinitywifi" (the app does nothing, and "XFINITY" refuses to connect due to "incorrect credentials" despite that I tried the exact same credentials on the iPhone and the second laptop!).

Why??