I'd like to keep it neat without anything sticking out and being visible from the cupboard. None of these devices had any issue with my previous computer that was placed in the exact same manner, so I imagine it must be possible to get it working here. But could it be that the network card is simply trash and can't output any stronger? In that case the same kind of USB BT adapter could simply be placed in the cupboard. I think that would be an acceptable backup solution if I can't resolve it with existing hardware.
EDIT: I should add, keeping the door open does not improve anything. The cupboard is wooden.
Well it was a quick and dirty solution that would've been easier to implement.
What kind of computer is it?
I have a OG Surface Pro tablet and the Bluetooth and WiFi will often fail because they're too close to each other and causes some kind of interference or something. Maybe there's something causing interference in your case? Wireless phones? Something else that uses a frequency in the same range?
It's a Minisforum UM690S. Currently I've turned off WiFi, and there are no other devices nearby with Bluetooth turned on. I have a laptop and phone nearby that are on WiFi.
I just tried to have a gaming session a bit closer to where I would typically sit. It would sometimes work flawlessly for a long time, but then it would enter a period of lag and missed button presses. I can't think of whether there would've been anything that started interfering at the times it started lagging. The keyboard can be pretty unresponsive very close to the computer as well. This keyboard has worked perfectly with another mini-PC at my desired distance that this computer meant to replace.