A few years ago I had a similar problem with Armstrong Cable in NE Ohio and went through a similar process that involved multiple unhelpful tech visits eventually culminating in me writing a scraper to read the diagnostic page and collect SNR/Tx power/BER stats per channel plus some investigation with a SDR to prove that the two channels having severe problems overlapped with one of the frequencies used by a nearby AT&T tower, and that I never had problems when I wasn't attached to one of those channels.
After all that they did some kind of RF leakage test and found that chipmunks had been chewing on the RG6 line from the street so that got replaced with the big thick orange cable and all was well for the rest of the time I lived there.
About three months later they pushed an update to their modems that changed the seed used for generating the random password that "protects" the advanced diagnostics page from the default so neither I nor anyone else on their network could access it anymore without inside information.
They went out of their way to take objectively useful diagnostic information away from us. A pure evil move, but of course one they could do with impunity because most of Armstrong's service area their only competition is Frontier's ex-Verizon DSL which was hot garbage 20 years ago and has only deteriorated since then. No one who would ever care about that feature is going to change providers over it going away, the sad state of the US broadband market means they have no real choice.