[Q] Unlocked AT&T RM-877 not getting LTE in non AT&T networks

Hello,

I have an SIM unlocked but AT&T branded RM-877_nam_att_205 with Lumia Denim Update:

Name: Lumia 1020
Model: NOKIA 909
Carrier: AT&T

OS version: 8.10.14226.359
Firmware revision number: 3051.50009.1451.1001
Hardware revision number: 3.3.0.2

I have a Movistar MX LTE SIM whichs works fine, even tethering works, but even it shows 4G connectivity it does not really connects to LTE, accessing the Field Test ##3282# in the LTE info says:

Sys Mode Invalid

Also in the field test app forcing the phone to connect with 4G only does not work, only with 2G and 3G only modes.

I know there is no way to flash my phone with another ROM other than AT&T one, and I really would not want to change the ROM, but only to get the LTE speed work with my carrier which has already LTE Band 4 enabled in my city.

Is there a way to enable the LTE connectivity without changing the ROM?