IMHO.Hey guys.
I just ported my kid's line from SIMBA to heya because the old phone needed SIMBA voice app for calls and it wasn't stable. After the porting to heya was completed, I lost data/calls/SMS. This is even worse than on SIMBA. APN settings and various data/network settings are correct. Asked heya support and was only asked to repeat whatever I'd done. Is there any hardware requirement for heya? To give more information, the phone was able to support SIMBA on its data and SMS, just voice calls had to be done via their voice app.
Take out the SIM. Boot up and delete all Simba Apps and APN. Switch off. Insert SIM and see if auto detect.
Also can try on another phone to see if SIM is working.
If not, your phone chipset GG.
P.S. I encountered 1 case before but opposite, phone after awhilr cannot connect to TPG but can connect to Singtel. I suspect becauseTPG signal weak, the chipset cannot sustain long term, died.