Hello, & a warm welcome to the Forums & Macdom!
Might try one of these...
Instead of joining it from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed. Before I went through network diagnostics and all of that stuff in system preferences, and then I tried the "Join Other Network" method, and now it works like a dream.
Make a New Location, Using network locations in Mac OS X ...
The Interface that connects to the Internet, needs to be drug to the top of System Preferences>Network>Show:>Network Port Configurations and checked ON.
System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.
The interface that connects to the Internet should be dragged to the top of the list.