I unlocked my phone. Wtf do I do to make it work right?

My family decided to switch to from AT&T to T-Mobile. Rather than buying all new phones I decided to stick with my Samsung Galaxy S5 (and GS3 as a backup), thus I got my phones unlocked.

I can make calls and browse the internet fine, however my phone does not get LTE anymore. Ironically, my brother who decided to get an all new phone with T-Mobile (HTC One M9) works fine with LTE. Since they are both GSM phones they should be compatible with the network? And I believe my phones are compatible with all the different LTE frequencies (I am not 100% sure on this though.)

Also, how would I get the Wi-Fi calling to work on my phone? I learned this is a feature that comes preinstalled on T-Mobile phones and there is no way of adding it on (easily.) So how can I get this feature to work? Would I have to root/flash my phone to the T-Mobile version of the OS?

Lastly, will I be able to update my phone in the future? When I go to update it, it says it is connecting to the AT&T servers.

TBH, when you get your phone from a specific carrier, it will work its fullest on that Carrier. That Galaxy S5 is what you got from AT&T, while you unlocked it to run on T-Mobile's network, but it doesn't have all the bands that support T-Mobile's network and that's causing you not to have any 4G signal at all. Have you tried going to the T-Mobile store and explain?

After a few days I've begun to realize I now to get LTE in select areas (of course it's not in my house). After a little bit more research I found out that t-mobile doesn't support all the different LTE radio bands.

But still how do I get the wi-fi calling to work? Calling t-mobile kept trying to get me to buy an all new phone...

That Galaxy S5 is what you got from AT&T, while you unlocked it to run on T-Mobile's network, but it doesn't have all the bands that support T-Mobile's network and that's causing you not to have any 4G signal at all.

I don't believe it will work on your current phone model as that feature was built for T-Mobile Galaxy S5's and not the AT&T's Galaxy S5. You should have just traded in your old phone for the T-Mobile one. Or next time purchase an unlocked phone that isn't carrier specific from the Google Play Store.