The router is not changing pass, someone or something with remote access is!
If you can access control panel, try searching 'connected devices' and see if you recognize any unauthorized IP's. (you should have a list of all IP's attached to your network)
You could enable MAC filtering which would only allow devices with an approved MAC addy (that you have previously set) access to your network. This would help bypass any unauthorized 'backdoor' access something may have to your system, at least till your next reboot.
Keep an eye out for 'multiple' copies of the same address as this may be a result of 'spoofing', which would suffice our suspicions.
Also, for PC/Laptop, download and install RougeKiller, reboot to 'safe mode' without networking, and run!
Reboot, D/L CCleaner, analyze then clean......do the same with registry. (no changes to default sets)
For mobile devices, install Malwarebytes for Android, iOS and Mac, disconnect from router and run.
All devices you connect to your router should run the above.
Edited by Replicator, 18 February 2018 - 05:39 AM.