I have this old 5s and have moved to the iPhone 6. So when I retired the 5s it would cough up messages about no SIM or no Service.

YOU have to decide what you want to do about this. For me I follow the CNET Cheapakate and obtained a plan with Freedompop for zero dollars a month. This allowed me to activate and use my 5s as a backup phone with 100 minutes talk, 100 text messages and 100 megabytes a month which autostops when the data is depleted.

I did look through all the apps and set them to not use my cellular data which works for me on this phone.