Hmm, same for me... anyway, if you have two SIMs installed, you have two separate places for these SIMs.
Also, only one SIM has LTE availability and there is no such settings for the other - that may be the reason it does not find it.
However, in Setings, under Network / Internet, there is a Mobile networks menu item and here are the two SIMs (sorry for incorrect naming).
One of them may have VoLTE activation checkbox.
Additionally, now I have VoLTE enabled for my company SIM, so I may have to check if it is working... I don't know when it was activated but I have used the other