AndzejJ, I have to apologize to you and cancel my earlier words.
After I reset to factory settings setup everytihing one more time, Passthrough works finally great and smooth without any hanging up and complaining.
Please chase other Team to include information in global changelog that RutOS 7.6.12 also includes fixes like below, it will be beneficial for other people:
Mobile: updated bridge/passthrough mode logic to flush ARP and add static ARP entry if MAC is specified
Mobile: added metric to default gateway when using passthrough mode
Now, reason why I complained was my fault - I switched in mobs1a1 interface “Subnet selection” from: AUTO (default) into P2P (which not works with my scenario it forces to use mask /31 not /30 I mention in the other ticket).
Anyway, after switching it to AUTO it works great.
Sorry for inconvenience and fault accusation.
This ticket can be closed. I will also update my other ticket to finally enclosure process it.
Kind Regards,
Robert.