Galaxy J3 Wi-Fi issues (Android 8.0.0) Expected functionality:
Wi-Fi works.

Actual functionality:
Wi-Fi sometimes connects and works fine. Other times, it will say "Internet connection may not be available." Sometimes it will say "Connected." but then the Internet won't actually work. It's not the signal range because there are 3 or 4 bars. My laptop in the same room connects to the Internet via Wi-Fi correctly.

I have tried:
Resetting network settings,
Setting static information and not DHCP.

It's a brand new phone.
Most probably this will be because of service providers issue, not from your phone.

I have seen this kind of behaviour mostly from jio network in India.
And all other network providers also now a days having this network glitches because of many criteria like tight fights in business, many changes in telecom regulations etc.,

Suggestion that I can give is place your phone in some good position where signal is best.
And you can ask your neighbors with same network if they are having internet speed issue.