Restore image help (Dell Wyse 5470 mobile)

I wonder if anyone can help me.

I have a Dell Wyse 5470 thin client which I use to connect to my home computer. I replaced the OS that it came with - Windows 10 LTSB/LTSC for Dell ThinOS 10, found it was no use because of the VPN problems, I use openVPN but it uses a certificate and password, not username and password (unless there is a way around that?).

I'm now looking to go back to Windows 10 LTSB/LTSC, the image I use on Dell's site doesn't seem to be applicable to my system and I tried a USB image writer but I get error 233 every time I try to restore the system. At least on Windows 10 IoT 2019 I think it is.

Dell's technical support won't let me contact them as it has come out of warranty, how do I get hold of a restore image in this case for my particular device?