

Yeah, I found that out when I was using my old phone with T-Mobile. It was listed as supported, and it mostly worked, but there was at least one LTE band it didn’t support.
T-Mobile uses multiple bands in the same area: one higher frequency, higher bandwidth, lower range one for fast data and a lower frequency, lower bandwidth higher range one to fill in the gaps. My phone didn’t support the lower frequency one so I would lose coverage if I was too far away from a window in my house (despite living close to 3 towers).
Not to mention, they can and do reallocate spectrum periodically so while you may be fine for a while, if they reallocate and the device doesn’t support the new bands, you may suddenly lose service.
Web app, specifically, or any apps?
I’m also familiar with these three (web) apps:
Might also also check out https://lemmyapps.com/ which has a good list that you can filter by platform/feature.