Cell phone coverage in Cuenca

T-Mobile doesn't exist in Cuenca. If you have lots of money to spend on international roaming with T-Mobile, then it will probably be $2 per M of data -- about $400 four hundred dollars per week. YIKES.

Claro or Movistar are your best choices for cell service in Ecuador. I run a business and $4 per week prepaid on Movistar is sufficient for my needs. I get 4G and 3G connectivity in most populated areas.

I suggest checking out Google Fi and see if that makes sense for you. Or read this thread for my suggestions on porting your number and parking it: https://www.expat.com/forum/viewtopic.php?id=783151.

I spent 2 months in Ecuador last year and used TMobile the whole time.  Texts are free, calls are .10 minute.  Texting works best with What'sApp or Facebook Messenger.  The service will mostly roam on the local services, Moviestar or Claro but service is good.  I was happy with TMobile while I was down there.  You have to have a OnePlus plan I believe.

How much did the TMobile OnePlus cost you?

AT&T has two options for international travelers users as of Jan 2019.
AT&T International Day Pass at $10 a day plan. And, the AT&T Passport, which starts at $60 per month, with 1G of data and $.35 per minute for calls.
Or wing it, like some people do, and rack up $1000s.

We are on a flat $60. senior plan.   Pretty good deal if your 60 or older.  It's less for one person.

Actually, for a visitor, TMobile is great.  I used it for 2 months with no extra charges, but I did not make phone calls.  I used text and Facetime.  Don't use it as a resident.  I agree with you on using Moviestar or Claro as a resident.

Just to clarify.  Lebowski888's answer is correct if you are going to live there for awhile.  Short term, TMobile works great.