I often find myself working in SCIFs (that’s Secured Classified Information Facility, for the uninitiated). No phones allowed. Unfortunately, my other self teaches in the evenings and I have a gaggle of students who live and breathe by text. I’m not sure about T-Mobile, but with Verizon Wireless, I’m able to text via web interface by logging into my account through the unclassified system on my desk (personal email is blocked, but text seems to have snuck in under the wire). Works pretty well, in my experience.
That’s the only nice thing I’ll ever say about Verizon.
This uses an app on your phone that communicates your text messages back and forth over the Internet. If you do not have good cell coverage (as stated in the OP) it will not help. (I use MightyText which is basically the same thing.)
Both users have to use Whatsapp. So it won’t work for Mrs. Devil who does not have a data plan, and as stated in the OP this is not the time for her to deal with a new app.