Hey C&MT, it's been a while for me. I'm putting together a passively-cooled Qotom system (Atom C3758R) to become a new OPNsense router... More accurately, it will probably run Proxmox, with the router ...

I'd return what you have, and get something that's on the QVL for your motherboard. Sometimes some RAM just won't play nice with some boards short of a lot of fiddling with RAM timings, and as you say ...