As of right now, with Mikrotik R11e modems in United States, you'll never get all the bands for ANY carrier. And miss some "carrier aggregation" and extended bands like 66 or 30 (or Band 14) that allow greater speeds. In general, an iPhone will have better speed than the wAP R LTE because of the lack of bands. So let's start there...
If your going to use the device in the same location, you can use
https://www.cellmapper.net – which helps a lot in placement and determine what carrier/bands you might need. Now if you might use in multiple locations, the summary is I'd recommend AT&T in the US (outside other factors).
Our experience is T-Mobile coverage isn't as good as AT&T. While in rural area T-Mobile sometimes has Band 71 (600Mhz) and we've seen no AT&T coverage – but Band 71 is not supported by the R11e series modems. So basically if you view the problem as "what has the most spectrum available on bands that Mikrotik supports" and you'll find the greatest overlap is with AT&T.
Verizon sometimes needs Band 13, which Mikrotik modems don't have. Since that comes up a lot, using Verizon isn't ideal. Plus, Verizon won't let you activate a R11e modem directly (you can move a SIM and it will work. So to activate a SIM, you need a 2nd device that is "supported". Basically can't give the Mikrotik IMEI to Verizon for the Mikrotik modems and expect them to turn it on.
If you're using the R11e-LTE-US modem, you want to make sure it's using the latest firmware which should have "v5" or "16.x" sometimes NOT "v2" or "12.x". The R11e-LTE-US isn't a great choice but does work – doesn't do carrier aggregation and has historically been buggy – so that should be avoided. The R11e-LTE6 supports carrier aggregation, with some the combos AT&T supports (e.g. 2+12/17), which is pretty common, so at least you'd have some shot of CA working which gets you more download. But you won't never get the 3D+2U CA combo with the R11e modems, which AT&T does support which give you more upload from CA.
I'd search for some of poster named "SiB" articles since he explains more on what do about this. We don't use Mikrotik's modems in the US as a result of the above – we use Sierra or Telit modems. The NEWER Quectel modems would be better, but yet available in the Mikrotik lineup.