MR1100 not connecting to T-Mobile towers despite T-Mobile APN

My home is on 50 acres. DSL won't reach, satellite is gunk (though I'm hoping StarLink will pan out whenever that order arrives), and I've looked into smaller microwave carriers in the area. Long story short - using a cellular connection is my only option. I've got internet through a small re-seller of T-Mobile bulk data plans, which has been fine but not without issues.

Originally we used an LB1120 modem - then that seemed to spontaneously give up the ghost and we were forced onto a RUT240, which is more feature rich but I hate because the UI is sluggish and the modem reboots itself regularly for no reason - not good when there are three adults that work from home.

So I grabbed an MR1100 - purchased straight from Netgear as an unlocked device. Firmware is the most recent from Netgear itself and is - to my knowledge - open to whatever you point it at, carrier wise. MR1100-100NAS_NTG9X50C_12.06.11.00_GenericNA_05.03

The SIM out of my RUT240 goes in fine, the MR1100 auto-detected the right APN settings for the T-Mobile network in the area, and says it's USING that APN, but when looking at the Status -> Diagnostics page, the MCC,MNC values are 310,410 - which my 'ISP' company tells me is AT&T, and this pans out by looking at 'scan networks' under the Cellular settings page and it has latched onto the AT&T/Verizon network despite the T-Mobile APN.

How do I crowbar the dang thing into actually using the correct network? If I change Network Selection from auto to Manual and pick the T-Mobile network by hand, it just doesn't do anything and those MCC,MNC values stay the same.

Netgear lists the MR1100 as 'works best on AT&T and T-Mobile', and T-Mobile says it should work on their network as well, but come hell or high water I can't get the MR1100 to actually USE the T-Mobile towers.

Model.json output below, it wouldn't let me attach the text file.

"general": {
	
	"defaultLanguage":"en",
	"PRIid":"23113828",
	
	    
		"activationDate":"0",
		"genericResetStatus":"NotStarted",
		"reconditionDate":"0", 

		"manufacturer":"Netgear, Incorporated",
		"model":"MR1100",
		"HWversion":"1.0",
		"FWversion":"NTG9X50C_12.06.11.00",
		"buildDate":"2020/01/24",
		"BLversion":"NTG9X50C_12.06.11.00",
		"PRIversion":"05.03",
		"truInstallAvailable":false,
		"truInstallVersion":"",
		"IMEI":"015240000383195",
		"SVN":"13",
		"MEID":"",
		"ESN":"00000000",
		"FSN":"5K150ANX506DB",
		"ATTDeviceId":"",
		"packageName":"MR1100-100NAS_NTG9X50C_12.06.11.00_GenericNA_05.03",
	
	"activated":true,
	
	"webAppVersion":"MR1100-100NAS_04.01.80.00",
	"HIDenabled":false,
	
	
	"truInstallEnabled":false,
	"truInstallSupported":true,
	"TCAaccepted":true,
	"LEDenabled":true,
	"showAdvHelp":true,
	"showWebInfo":false,