I searched all the online solutions already and did my due diligence (reboot the phone, reset all network settings) and nothing works.
I tested 2 Wi-Fi networks in the area with a laptop. One of them works fine on the laptop and fails on the phone with "No internet connection" or "unable to join network". The phone can't see the other although I can see it with almost full signal on the laptop.
As mentioned in the title: iPhone type = 6s, iOS version = 11.2.6.
This Wi-Fi (mis)handling on the phone is sloppy and gives up too easily. I expect better. Bonus points for answers with configuration info to keep reconnecting automatically with no input from the user.