Z Fold 3 using from Hong Kong, with same issue, after update One Ui 5.0 to 5.1 and the phone restart it could not go on Wi-Fi. Tried "General Management" "Reset" "Reset network settings" did not work, tried "Factory Reset" and did not resolve it.
Brought it in to service center and they told me the Wi-Fi module is damage and need to change it, but because its link to the LCD it will need to be change as well (repair cost 4200hk$ 535US$) after 2 day back and forward they told me they give me 50% off the repair cost, as I did not want to wait anymore I paid for the new quotation and now waiting for my phone to get repaired because I press update 5.0 to 5.1 lol.

I have currently find around 10-15 case that involve Z Fold phone and One UI 5.1 – Wi-Fi problem. Please Reply post if you have this issue. Thanks in Advance.