I had opted for volte in settings but it is showing need to register, checked with network provider, they are showing this phone does not supports volte.
As per the screenshot I guess you are on the latest software version.
Can you try swapping SIM from SIM 1 to 2 and vice versa. Also check with other phone if SIM is VoLTE enabled.
Try switching Roaming On it may help in getting the VoLTE enabled.
I can confirm that Airtel VoLTE works on ROG Phone 3 as I am using the same.
If nothing works try backing up your phone any factory reset