Works flawlessly on 4G, really badly on WIFI

After months of patchy Wi-Fi operation and lots of rebooting, resetting and even replacing the unit, I’ve discovered that my bulb works perfectly over 4g.
I guess this would point to a problem with my router somewhere but I feel like I’ve tried everything.

Can anyone think of a reason why this would be the case?
I’d write a list of what I’ve tried but this would be too long.

Many thanks