Am not receiving sms text messages

For some inexplicable reason my iPhone has stopped receiving sms messages — for example verification codes used to cash in Bing points — I assume this means I’ve also been missing text messages from friends/family who either do not have an iPhone (Heathens!), or who are out of the country, as my children have been for a month.

I have searched (and researched) online and attempted all the solutions that have been put forth — turn iMessage and FaceTime off, reset network, reverse the process; be sure address contacts and other numbers are not blocked; ensure iMessage toggle is on ... — all to no avail.

I have an iPhone 5s

Current software is 11.3.1

AT&T (4G)

I have no wifi at home — though as a technology layman I have no idea what, if any, affect this has on my issue and using the WiFi at the library altered nothing.

I suspected that the graying out of my number in the Send & Receive area had something to do with the issue, but posters online said it should look like that.

I am desperate and really need this fixed.

The only thing that has changed recently is that I got a new cell carrier and thus, a new SIM. When I phoned them for support they said to reset the network (did that twice and still nothing).

Just prior to changing carriers I had gotten an sms text verification code for something and received it without a problem.

Please please please is anyone able to help me?

Thank you in advance for any and all assistance.

p.s.

Please (please) do not suggest that I upgrade to a newer phone. I have neither the resources nor the interest as I adore my 5s.