WLAN works in High Sierra, not on Big Sur

So I have an iMac that won’t enable WiFi (greyed out) in Big Sur. I booted externally from on old High Sierra external SSD and it had the same problem, so I reckoned it should be a hardware issue.

I replaced the WLAN adapter (PN: Z653-00011) with a WLAN adapter that belongs to the bigger 27” iMac (PN: Z653-0005). The replacement module is a bit smaller but it fits in the board and the antenna’s snap on nicely.

Now the wifi is no longer greyed out but it finds no networks. However booting from High Sierra (external ssd) the WiFi works not just nicely but even with impressive speeds.

I go out of my way and re-install Big Sur on the iMac, still no networks. I update to Monterrey, still no networks! SMC reset, NVRAM reset.. nothing. What the !&&* is going on? I’m getting tired and hope to get some advice.