To do that, I needed to have the stock UI working, however I figured it out.
I used the M7350 MiFi firmware as a scheme example to understand how the firmware scheme looks like, downloaded the MR200 Firmware and extracted the modem firmware, since OpenWRT allows logging in to the Modem Console (192.168.225.1), I have went to the Device Settings and have done a Firmware Update, which unlocked the modem.
Now just need to install a TP-LINK MR200 firmware back to be able to properly use the modem and we should be good to go.
Full Guide
https://forum.openwrt.org/t/guide-on-unlocking-tp-link-archer-mr200-from-your-carrier