End of 3G forcing me to change from Cat S60 phone. Will Cat S61 or S62 work?

I've been using a Cat S60 phone with AT&T Prepaid for almost half a decade now. I just got a text message yesterday that they will no longer let me place calls with it after tomorrow (thanks for the advance notice guys, would it have killed you to give me a couple days extra?), because even though the S60 has the band capability for 4G calls, for some reason it is only allowed to call on 3G.

I work in construction and industrial service, and the integral thermal camera has been extremely useful, as has the waterproofing, impact resistance, and heat/cold resistance. Nothing else commonly available seems to meet the same specs as the Cat phones, and if I'm being forced to upgrade, I'd like to go with either the Cat S61 or S62. 

I've got a Galaxy S9 coming on Monday, I plan to go to the local AT&T store and switch to postpaid, because there are apparently differences between the two services (prepaid customers not allowed to roam on other towers?), and they won't let me set up a postpaid account with a "non-approved" phone. Once that's done, I am hoping to order an S61 or S62 and simply swap the sim card out and gain the features I want back. 

Does anyone know whether or not this will work? I see T-mobile is actually offering the S62 now, so if not I'm seriously considering switching to them because I liked the phone so much, even though the coverage might not be as good.