Unfortunately this ISP sucks, and it’s part of the contract.
https://www.ecfr.gov/current/title-47/chapter-I/subchapter-C/part-76/subpart-P/section-76.1201
https://transition.fcc.gov/Daily_Releases/Daily_Business/2016/db0510/DA-16-512A1.pdf
It's unenforceable. FCC has mandated that users can supply their own modems and routers (including if the modem is provided for free). They must allow it. You can start an informal complaint with the FCC if they're claiming that you must use their equipment. Usually gets the companies gears turning on the matter.