OK. It sounds like you want a Wi-Fi network in your RV. Lots of companies provide that. Pepwave is one of those. Winegard is another. Cradelpoint comes to mind too.
All, as far as I know, all require a SIM card and some sort of cellular plan. The cellular part of that can be considered your 'modem'. Those 'permanent' devices are basically just cellular modems with WiFi routers.
As far as taking the SIM card out of your phone and putting in a cellular modem/Wi-Fi router, that would render you phone useless. Not sure that's such a good option nor am I sure T-mobile would support your phone's SIM card in another device other than a phone.
You'll find lots of folks using a lot of complicated and technical set ups (including me). I'm a pretty technical nerd so I'm OK with that stuff; but I have chosen to use my phone as a hot spot 95% of the time and invested in a cellular antenna/repeater after years of trying different approaches (it's kind of the KISS principal). I only use it when the cell signal is weak, it works well as most campground's Wi-Fi is woefully inadequate. When I do find a campground with a robust network with plenty of bandwidth I have
this arrangement (also very technical).
btw - that link is one of David Bott's, I find he is a good source of technical stuff.
and you might want to do a lot of viewing/reading on this YouTube channel......
Mobile Internet Resource Center There is A LOT of good advise there but sometimes it is overwhelming.