Come across this one all the time when I get a curtain job...
There's a thing called a ''wet patch''...I get mine from Wickes...Screwfix do them as well ....you usually find them among the fixings.
It's a circular piece... of what looks like plaster of paris material...around 2 .5'' in diameter.
Scrape away all the loose and broken plaster...making sure the hole is clear....
You just get a saucer of water...dip the patch in it..put the patch over the end of your raw plug and push the whole lot into the hole....simple as that ...if the whole is large then use two of them...one on top of the other.
Go and put the kettle on and make a brew....when you get back..I promise you...it will be ready to screw into .....all dried out, and a really strong fixing..
They are really an amazing bit of kit....I've always make sure I've got loads of them in my van..whenever I get a curtain job..!