Not SCB but I use the Kasikorn app in the UK. I have a True SIM in the phone and roaming enabled but NOT data roaming - that's spoils money incredibly fast. Data is either WiFi or the UK SIM. No problems using the app at all. I would hope/expect that SCB will work similarly. No special amount of credit is needed for enabling roaming, as long as you don't answer the phone or make calls/texts. It will receive text messages free of charge.
Presumably the app is up and running on the phone already? I mention this because at least in the case of Kasikorn the initial setup has to be done while connected to data on a Thai cell network, not WiFi.