1. Go to 
Settings > 
SIM cards & mobile networks > Advanced settings.
2. Manage advanced settings.
Use mobile data during VoLTE calls on the secondary SIM: During VoLTE calls, the phone connects to a mobile data network with the secondary SIM. But you can set the feature when you insert two SIM cards.
Data roaming: When roaming, you can connect to data services.
Settings > SIM cards & mobile networks > Advanced settings.2. Manage advanced settings.
Use mobile data during VoLTE calls on the secondary SIM: During VoLTE calls, the phone connects to a mobile data network with the secondary SIM. But you can set the feature when you insert two SIM cards.
Data roaming: When roaming, you can connect to data services.