Are there any options for a T-mobile iphone 5s?

The phone was bought at an Apple store, so there's no other issues. Getting a new phone is out of the question - it's my mom's, and she hates the idea of buying another phone.
And it'd be best if it has unlimited 2G.
Thank you so much!

mintsim $199 a year unlimited everything wih 2gb lte. Unlimited data is at 128k which is very usable. Unless she has very little use nothing comes close.

What kind of usage does she anticipate?

An LTE SIM from FreedomPop might include enough data to not need unlimited 2g.

What's her budget? That'll help rule out some possibilities.

Talk and text, next to none. She receives a few texts, subscription from stuff like GAP/Sears/whatever memberships, you know, nothing important. Sometimes she receives a few calls, average less than 1 min/day I think, nothing super important either.
Cheap/add-on/unlimited calls to China would be very nice, but not a strict deal breaker if there's a super good deal.
Budget, under $20/mo (that's what we currently have, no point going above that).
One thing that makes it harder, is usage will be intermittant. Say, she uses the number for a few months, then stop using for maybe half a year, then use it again for a few months. (If something like 1 call/text every month is necessary, I guess I can put the sim on my iphone SE to make a 1 minute call.)

One more question.

Why would unlimited 2g be a factor?

Might be perfect for freedompop. Mint might be overkill.

She uses a messaging app to get in contact with people, and she would send pics, voice, things when she's out for walks, and she needs Google maps sometimes. I just don't want to tell her "hey, mom, watch your data use, if it goes above x MB you'll have to stop", that's too complicated for her. So to save the hassle I'll just get unlimited 2G (which is what both of us have now). I, on the other hand, am an actual heavy data user. It's easy for me to go above 1G or even 2G a month. But I have a sprint conpatiable phone, so I'll just get Tello for now.

If you need 2g then Mintsim it is.

I tried FreedomPop. It says "We're experiencing an issue with your provided device ID. To complete your activation, please call 800-671-5017."
Since the web link was I assume they can't do T-mobile devices.

If you want to do freedompop you need to buy one of their SIMs - either the global sim or the ATT LTE sim.

The global sim is hsda only but uses both tmobile and att. The ATT LTE sim is ATT only but includes LTE. Your tmobile iphone should take either.

DO note both are VOIP only for calls.

Nope... see.