If you’re losing network connectivity and you can’t install a console directly on the machine to see what’s happening, it’s more or less game over.
You can try and look at the WiFi AP/router log files, they might reveal something.
On the Pi, “dmesg” will print kernel messages some of which might not be reflected in the system log.
Otherwise with /var/log/syslog on Raspbian you have a comprehensive view of what’s going on.
Applications such as wpa_supplicant allow to specify log verbosity. This one in particular can be very verbose (and cryptic)...
Also, keep in mind the unpredictable nature of wireless. If the neighbor next door has changed WiFi settings on their router, your network might be affected.
"S'il n'y a pas de solution, c'est qu'il n'y a pas de problème." Les Shadoks, J. Rouxel