Model R6020- Connected with great strength but internet won’t work

> I just moved into an apartment that provides internet and cable. They
> left me with the modem, router, and cables. [...]

   Did "they" also leave you with a service agreement with your
(cable-TV) ISP?

   Does your "the modem" have a maker and/or model number?

> Model R6020 [...]

   Firmware version?

> [...] I've tried everything. [...]

   I guess that there's no hope, then.  In that case, why ask here?

   Have you tried asking your ISP?  Or the apartment management?