Following the long thread on the same subject discssing the BETA FW, as I said I would, TPLink have given me permission to share the FW download llink.
From TPLink - We recently released a beta firmware for Archer MR600(EU) V1 which can allow us to select 4G+bands for Archer MR600 manually.
For Archer MR600, if you want to use its 4G+network, your SIM card should support the CA bands group Archer MR400 supports. Please check the attached picture and check if your SIM card supports the band groups on the list or not.
They asked me to state that, this BETA FW is for the Archer MR600(EU) V1 ONLY!
Neither TPLink nor me take any respnsbility if you flash a none-EU router.