2011 Macbook Air connects to WiFi but can't get Internet This 2011 MBA is updated to High Sierra.  I have tried to connect to 5 different WiFi networks but in all cases, the MBA can connect to local network but no Internet.  

All 5 of these networks are professionally configured in hotels.

Running Diagnostics seems to say everything is ok.  Also, at the same time I can connect to the same networks with another Macbook Air (2013, Mojave) and two different iPhones (8 and 6s) with no problems.

If I turn on Personal Hotspot in either of the two iPhones, the 2011 MBA obtains Internet no problem and gives a fast connection.

The 2011 MBA used to work fine on wifi with no problems.  It may be that updating the OS has somehow caused a problem, but I'm not the primary user of the 2011 machine and am not sure if the problem appeared with HS or was there before.

Does anyone have any suggestions to troubleshoot a WiFi connection that doesn't get to the internet?
As you say this only happens with Hotel Networks.

Usually the process with Hotel networks is 2 steps.

1- First you log in to they WiFi

2- Once logged in, the Hotel Server will send you a "Captive Portal" web site, on which you must continue to get Internet access. I assume you do not get t this step (no portal showing)

Your Mac (for your protection) before that popup shows, will check the certificate revocations to verify the popup being shown isn't a fake one.

Fixing it:

Usually there is simple fix to show the Captive Portal by just entering cnn.com in your browser and it will redirect you to the Captive Portal, but not always.

Try with different browser open.

Alternative solution is to create a "new location" in the network system preferences window, name it Hotels if you whish, and remove all DNS servers.

Similar problem and solution described here https://apple.stackexchange.com/a/37851/46541