Please correct me if I am wrong. You are using regular SIM card and after someone calls router's number, you connection stays in 3G mode forever?
If that is true, it is not normal behaviour and your SIM card should automatically switch back to 4G after call is ended.
Try to set manual APN settings by navigating Network > Mobile and General tab (you can find your APN in the internet or ask your service provider). Also you could try to enable Force LTE Network feature (Network > Mobile - General tab).
And to answer your question about VoLTE, you can't set status to VoLTE if your operator does not provide this feature for your SIM card.