@Kevin_Z 

Thank you for your reply!

This is EU version of MR6400.

This is the version string from the web interface:

I downloaded the old firmware from this link: https://static.tp-link.com/2020/202011/20201116/TL-MR6400(EU)_V5_200511.zip
And of course, unzipped, and the tried the upgrade with the bin file, from the zip:
filename: TL-MR6400v5_1.1.0_0.9.1_up_boot(200511)_2020-05-11_12.28.03.bin

length: 54610117 bytes

Do you have any suggestion?

Thank you!

@Kevin_Z 

Thank you for your reply!

Yes, it seems, cannot be downgraded. Great (no).

I tried wth two different sim card, the problem was the same (Error code 1. Internal error), no screenshot available at the moment.

I read the posts you suggested, but I cant configure profile while the SIM card isn't recognized.

I bought a new router in the nearest store, the same model: MR6400, EU, v5.0, I put my SIM card, and works perfectly. A tried with another sim card it works perfectly also.

It seems to my SIM cards are working, I suppose that the first router is faulty, and the firmware upgrade made it worse.

I hope I can send it back to Amazon and I get my money back, despite the firmware upgrade.

But if you have have any other suggestion, I would be happy.

Thank you!