My online banking, both website & apps, were working fine last time I left Thailand. Covid-19 has kept me away & during that time, things have changed, firstly, my iPhone XS Max died & had to replaced, Thanachart merged with TMB bank to become ttb requiring a new banking app, without which, I wouldn’t have been able to gain access to the ttb online banking website & finally, most recently, I bought the new iPhone.
On a side note, it’s far simpler to transfer UK banking apps between devices - no need to be in range of UK mobile masts &/or using mobile data from the UK SIM card.