Hi, I am new to Apple, I purchased a 2nd hand iMac 27" with an Intel Core i5., running High Sierra 10.13.6. All seems well but I am having some connectivity issues with my wifi. When I first boot the mac connects to my wi-fi using my saved credentials. However if I put the mac to sleep (or it sleeps after inactivity) then I'm unable to re-connect. I get a message: "The wifi network "xxx" could not be joined" - try moving closer to the wireless router.... I have tried manually re-connecting, removing the wifi and re-creating and re-entering credential but it will not reconnect. The only solution is to reboot again which connects automatically. Any ideas? Thanks in advance.