How about changing some wireless settings on the modem, such as disable wps, change channel to 1/6/11 and channel width to 20MHz.

Never meet the same issue before. Where do you see this error? Any screenshot?

Try this firmware? http://static.tp-link.com/Archer%20C3150(EU)_V2_170904_beta.zip

How about the wired connection of the modem? If all clients including wired clients drop internet at the same time, check the dsl light and internet on the modem. Better to upgrade the firmware if it...

The parental control won't take effect when it is disabled. It seems that the device has some special settings on it. Restart the device and check the IP address on it to see whether it is normal...

Here is a beta firmware for C1200 V1 EU version. http://static.tp-link.com/C1200v1_eu-up-1.0.2 Build 20161128 rel.53072.zip