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

6) Reboot

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