@DUDU-Meng So, I've resolved this:
UK_Matt
The driver for mobile data in 3.7 update is working differently to 3.6 and previous releases. I tried a full factory reinstall from USB and it fixed some issues, but not data. I had a sense it was related to the signal strength or protocol (dropping to 3G UMTS from 4G LTE or maybe even trying to access 5G NR which the driver supports but hardware doesn't). In Android the driver only offers 2G/3G/4G Auto - no option to fix a specific option. So, I used the hidden Android Testing menu (install Google Phone app - type *#*#4636#*#* into dialer) and in Data and Network menu set Preferred Network to LTE Only (note: other countries may need to choose another option). Mobile data works perfectly now - like it did on 3.6.