I have a CRS109-8G-1S-2HnD WiFi modem at 6.42.6 (I'm going to upgrade it here in a second). It's worked great for a few years now, but recently all WiFi connectivity drops. By rebooting it the connectivity is restored. The oddest thing I noticed after rebooting this morning was that the logs stated that the date was Aug. 20, 2018 (it was already the 21st!). And then the date and time reverted to Aug. 13, 2016! Here's the sudden change in the log:
Time Aug/13/2016 12:59:13
Buffer memory
Message cloud change time Aug/20/2018-21:44:29 => Aug/13/2016-12:59:13

I manually changed the date to the correct date and time, but why would that have happened? Is it somehow related?
Also I haven't checked whether there is internet connectivity through a wired connection when WiFi is down.
Anyhow, since changing the time to the correct time, it hasn't changed. Any ideas would be appreciated!

Remove the checkmark for "IP Cloud update time", it is a lousy time source.
Instead, configure System -> SNTP client with addresses of NTP servers of your ISP or alternatively enter the name pool.ntp.org in the name field.