TL-MR6400 v.1 - How to set LTE-module to one Band

Hi, All.
I have TL-MR6400 V.1 with OpenWRT v.19.07.5.
I cannot set 4G to one particular Band.
New V.3, V.4 can set fixed 4G-band via original firmware.
V.2 has three various LTE-module-firmware versions for 1800, 2600 and 1800+2600MHz.
What is V.1?
Does anybody know adb shell commands to set LTE-module to 1800MHz and 2600MHz ONLY?
The router looses the connection very often and reconnect time and again without this option.
Please, help.