We've had customers return them to us in the past because of similar reasons. The first thing we do is grab some random CFW, and it works absolutely fine 99% of the time (the only times it doesn't are if the device miraculously develops a hardware fault during transit because it got bumped around
).
Forgive me if I assume that there must be
some kind of error being made at some step.
Other things that have come to mind:
1) Is it being inserted in the correct slot? IIRC, for the RG351MP, the INT card slot (the one on the left) is the one used for the firmware.
2) Do you have any other card readers you can try using to perform the burning operation? Maybe the one you're using might be messing things up in some weird, esoteric way.
3) Tripl- no, quadruple check you're following the installation process correctly.
For every firmware currently available for the RG351 family, the process is broadly this, no exceptions:
-Download and extract the image file for the custom firmware
-Insert your microSD card into your computer.
-Use a flashing tool to burn the image into the card
-Put the card into your device (in the correct slot) and boot
Make sure you don't have any programs running that might be "automatically cleaning the SD card" or something like that during any step of this process.
You can also always go hop onto the CFW teams' discords to see if they have any more information to bring to light.
And on the
chance you're really doing everything correctly and that ONLY the stock firmware is working on the card (especially because you've tried flashing the stock on other cards without issue), we'd advise contacting the seller to see what potential options there are.