I have been using T-Mobile's 5G internet for close to 3 years now.
A couple of takeaways:
Your internet data is essentially what's left over from their phone data. So during heavy traffic your speeds can suffer significantly.
The "Unlimited Data" they advertise all the time was not made available to me immediately. There was like 100/150/200gigs of bandwidth monthly plans. I'm not sure if they would have ever offered it voluntarily, but I went to the website options to opt into e-mail notifications several months later and saw I could switch to the advertised unlimited $50 a month plan.
Quality-wise, it was decent for like a year, then there was roughly 6 months where it was quite unstable. A lot of disconnecting or speeds where it was essentially unusable. After that, though, I haven't really had any issues outside of the rare ones that all providers have from time to time. Currently my download speed is 240mbs, but like I said, it can fluctuate. I think part of why my speeds are much higher than advertised is that I live in a very rural area, so there is a lot more leftover data than other places may have. The "gateway" as they call it only has 2 ethernet ports, but you could always purchase a router. I don't know anything about their landline support, though.