Heh, a few months later and I'm banging my head into this same issue (reference: Waveshare Dual Ethernet 4G/5G box build - WiFi not working.
The strange thing is... this time I see there already is a firmware file for the compute module in my fresh build from the master branch of openwrt:
root@OpenWrt:/lib/firmware/brcm# ls
brcmfmac43143.bin brcmfmac43455-sdio.clm_blob
brcmfmac43236b.bin brcmfmac43455-sdio.raspberrypi,4-compute-module.txt
brcmfmac43455-sdio.bin brcmfmac43455-sdio.raspberrypi,4-model-b.txt
So I can't quite figure out why I can't get a wireless interface to appear.
Follow-up issue: Can't get WiFi interface working on Raspberry Pi CM4