iPhone 6s no LTE, only 3G super slow. Weak signal to "no service"

Right after boot it shows "no service", but after like 10/15 seconds it eventually get signal but only 1 or 2 bars.

No LTE/4g, just really slow 3G that takes one minute to load a web page.

Signal eventually drops and go back to "No service".

IMEI is shown with no problem. Phone calls and Wi Fi are ok.

What I’ve tried so far:

* Using 3 differt SIM cards with different carriers

* Network settings reset

* Initialize

* Change this antenna iPhone 6s Cellular Antenna

* Change the 2 antenna coaxial cables in the motherboard rear

I saw a vid where they change the U_WTR_RF IC, that could be the problem? How can I troubleshoot it? Have patience I'm new to this :-)

Thanks