Pretty easy. Just click an amount you want to use to refill your account. The higher the amount the longer it will stay active. 20 baht adds 30 days. And you don’t lose the money. Because in the app you can buy packages with data.
If you opened your bank account long time ago they will not just close your account. You had to make sure before 1st of may I believe, that the name of your Thai phone number matches the phone number in your bank account. Maybe people did not take care of that.
According to the Facebook page of the Bangkok Community Help Foundation, Bangkok Bank has reportedly begun freezing accounts and blocking the cards of Russian nationals who fail to meet the new eligibility criteria.