Even if you check the Dual 4G Networks that doesn’t actually seem to do anything so you to go into the secret menu and enable it. I wish it just worked out of the box. It probably will eventually.
1) Go to the dialer and enter: *#800#
2) Select Enter
3) Go to Logkit
4) Swipe down to function switch
5) Tick VoLTE and/or VoWiFi switch
7) Once that is complete, you can toggle the “Dual 4g” networks switch and actually get the Wifi-Calling & VoLTE options available for your 2nd Sim.
This will actually allow you to get Dual-VoLTE & Dual-VoWiFi(wifi calling) with your two T-Mobile sims