How to add modprobe command to GRUB_CMDLINE_LINUX_DEFAULT

Ubuntu 20.04
To make my WiFi happy I enter the following modprobe via the terminal;
modprobe -r brcmfmac
Then wait 5 seconds and modprobe brcmfmac
Can I add this to a startup process so that I don’t have to manually execute it each time?

I have tried adding it to my GRUB_CMDLINE_LINUX_DEFAULT but after I grub-update and reboot my wireless still hangs untill I manually enter the commands as aboove.
Is my syntax wrong? Can someone advise the proper way to add it?
Or should I not be adding this to the boot image?
I get the same result when I boot to my fallback 5.4.0-40 kernel image.