Flash N. American ROM to European Model for 4G in the U.S.?

[jchadana]

I live in Europe most of the time but am periodically back in the U.S. for months at a time. The different OnePlus 3T models support different 4G bands.
I'm wondering if anyone knows whether it is possible to flash a ROM for the N. American model to a European model in order to get 4G support in the U.S.? In other words, are the 4G bands a matter of hardware or software?

 

[pitrus-]

There are no different ROMs for American, Chinese and European OxygenOS. Only difference is in the factory bundled encrypted modem partitions which contains the IMEI and regional band frequencies lock-down as to not anger military or other organizations that may be using those bands in those countries.

 

[Anova's Origin]

Network support is mediated by a mix of hardware and software. Unfortunately, flashing a ROM will not be able to enable greater comparability. It supposedly is possible to flash radio images, which typically isn't changed during ROM updates, though it is a bit sketchy and can only unlock a few bands. You can read up on it here:
https://forum.xda-developers.com/oneplus-3/how-to/guide-unlock-additional-lte-bands-t3519563

 

[ElektroDancer]

Some bands are in both. Maybe it's enough for you.