Do you know how to determine the version of the tp-link tl-mr640 device?
Is it printed on the rear side somewhere? Or is it shown in the device software somehow?
I am asking because I am trying to get it on ebay, but the guys cannot tell me if it is v1
Cheers, Frood
PS: If there is any better LTE device you would suggest to buy I would love to read/hear it.
Better or worse is a matter of personal preferences. 
In general I prefer LTE router/modem with bridge mode. I have a Zyxel LTE3301 (CAT4) and a Zyxel LTE4506 (CAT6), both offer bridge mode. That way I have the free choice of every router, which is supported by openwrt.
The LTE router's LAN is connected to my openwrt's WAN (a rpi4b). This avoids double NAT.
As far as I know the MR6400 does not offer bridge mode.
For hardware recommendations please open a new topic in the Hardware Questions and Recommendations category of this forum.
Please see https://openwrt.org/faq/which_router_should_i_buy before asking for recommendations. This way you will get better recommendations and come to a quicker solution.
If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.
This sounds really the best. - I choose LTE4506.
For a second (HA) WAN, a bridged modem / router is the best choice. I agree.
Thanks & Cheers!
Frood
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