Why can't I connect to a particular WiFi?

I can easily connect to the Internet at various WiFi cafes, libraries, hospitals, dental offices, etc. The one place where I have a problem is my favorite WiFi cafe.

Sometimes it connects, sometimes it doesn't. If I can connect, then I can usually reconnect when I go back the next day. But if it suddenly disconnects, then I might be shut out for days or weeks.

Is there some way to troubleshoot this? Or can anyone suggest some common fixes?

I have another problem that might be related, though I don't think it is.

I recently bought a smartphone which I can use as a hotspot. It worked fine until one day it slowed way down. My connection is now so slow, it's almost worthless. Is this something an Apple tech could diagnose?

Than you.