3G is turned off on all US carriers. VoLTE (4G) does not work for calls or messaging. You won't be able to receive MMS, only SMS. Your phone should function on 2G. This requires a setting change in Ubuntu Touch where you turn off 4G support.
Data should work on 4G, but not calling or MMS.
Data will be slow on 2G, but you can make/receive calls with lower quality sound and some delays. You can also receive SMS text.
Ubuntu Touch currently does not support VoLTE (4G) or higher for the Google Pixel 3a. Similarly, your carrier may or may not allow your Google Pixel 3a on the network. You will need to check with your carrier. If the carrier accepts your phone, only expect 2G calling and SMS. Calling and SMS may have some delays. 3G service is not available in the US. If you need faster data and are not expecting calls, switch to the 4G setting in Ubuntu Touch.
I've tested the Google Pixel 3a in the US with a 5G SIM card.