Cellular Data Dropping While in Transit

I've had this exact problem. I'm from the UK and my mobile network provider is called Three Mobile. When in the USA my phone locks onto their roaming partner which is T-Mobile. Last week I went to Seattle, Chicago, Washington DC and Philadelphia and after random amounts of time my phone would drop the data signal (4G(LTE), 3G or E) but the little bars would still show sometimes and sometimes not.  I was receiving a Cellular signal (it just seemed to be the data dropping although I never made any calls to test this). This happened about 20 times per day every day in each city mentioned and was 100% annoying. From leaving my hotel to getting to a restaurant 200m up the road it would have lost all data signals! I have a new(ish) iPhone 12 Pro Max. The annoying thing is my girlfriends iphone XR (older phone) picked up the data fine and it never dropped once. I tried her sim in my phone and it did the same thing so not my home network (she's on exactly the same network and tariff as me). I tried my sim in her iphone XR and it worked perfectly so don't think it's a sim issue. I tried switching off 5G in the roaming settings and it didn't make a difference. I reset my network settings in my iphone 12 and it still didn't work. I have now completely erased my phone using a computer so that it has to re-download the whole iOs operating system (rather than resetting it from my phone). It's annoying for me as need my phone for travelling so can't test it until I'm next in the USA which could be months away and my phone will then be out of warranty (if it's the phone that's faulty). Apple suggested I buy a sim to use just in the USA on a different network but I don't want to have to pay extra as with Three I can use all of my included minutes, texts and data when in USA and all of my previous phones have worked fine there when data roaming. I switched off automatic network selection and tried selecting AT&T and Sprint but my phone won't allow this as I'm roaming and Three is partnered to T-Mobile. I have noticed in my home country that the 4G (LTE) signal seems to be worse since they starting rolling out 5G. After reading about T-Mobile and Sprint merging I wonder if it has anything to do with that? I was convinced it was my phone but now not so sure. I was reading about channels and someone wrote that the 5G opens channels that some networks can't use and tries to force your phone onto so drops the data signal but not sure. It's just frustrating that a phone less than one year old can't do what a phone from a few years back can! The airplane mode 'fix' works but it's very annoying in this day and age having to effectively reset your network signal just to be able to receive data!