-kindly ensure SIM cards are working fine with other phones
-reset the APN / head to settings → mobile network → Access Point Names (APN) → Reset the APN
-restart the phone
-select a mobile network when prompted
I have the same problem. Resetting the APN did not solve the problem. Stil I am not able to switch on "Dual SiM always on". The SIM 2 failed. It would be very useful if anyone can help me (us), because I see that there are many people with the same problem.