I was having problems with mobile data through T-mobile in the US that I was have to disallow 5G or else the carrier connection would drop. Sometimes it would reach 4G+, but it would drop the mobile data connection all the time, especially when moving off wifi. I had never seen the VoLTE icon. After I followed the steps below, the VoLTE icon appeared, 4G calling became a settings option, and the mobile data has been working smoothly. Basically I followed this guide, although I wasn't able to...Read more