Smart plugs again - tried 2.4ghz - any other tricks - do any work?

hi all

i've read up on this as best i can, im not a complete computer idiot but not a tech by any standard!!

Usual story here, upgraded to Fibre a few weeks ago with the latest modem/router, my smartplug wont work now, disabled 5ghz, still wont see the plug, its a cheap Ultrabrite one but it worked fine before, is this something to do with the 802.11ax thing? i saw someone suggest it is, will it work if i put that back to the oldest version available on the settings? will that mess up my network connection or speeds? 

If i cant find a fix is there any smartplugs that will work, i dont fancy the idea of connecting another device to the modem to make it work, I only use one plug to turn on my coffee machine either on demand or with a timer in my smartplug app, it needs to preheat, so with the smartplug it can be piping hot when i get out of bed or get home, i need my coffee!!!

many thanks