SurPad and SD card expansion issues

Hello everybody, I tried to expand the internal memory of my SurPad using a 128 GB SD card. When I use the external card as ‘portable device’ everything works perfectly. When instead I try to use it as expansion of the internal memory, something strange happens, when I turn on the tablet, I receive a notification saying SD card missing which disappears as soon as I unlock the device (this only happens after starting the tablet and not when I simply unlock it). Furthermore, even if on settings_storage I can see the total space (approximately 250gb) when instead I open the file manager, I can only see 128 GB of internal storage. I tried to use a different SD card but the problem is exactly the same.
Has anyone experienced the same problem?
Thank you in advance. Andrea