High Sierra 10.13.6 and wifi

Hi! I have a problem with wifi being dropped ONLY when my iMac has been sleep for some time. If I step away from my computer for an hour, maybe 2, it wakes and connects. But after having been sleeping overnight, it disconnects from the wifi network. I had resolved this with an apple tech 6 months ago, but it's doing this again. I have reset the PRAM, and SMC also. HELP!