@wagego 

Does this issue only happen on wireless devices? Please check if the devices that connect to the router via an Ethernet cable will work flawlessly when there is no internet on the wireless devices when the issue happens. If the wired connection always works fine, try to modify the wireless settings on the router, such as wireless channel, channel width, and security, you can check the guide below:
https://www.tp-link.com/support/faq/722/

When did you purchase the router? You can try to reset the router to factory defaults then reconfigure it from scratch, then confirm again if the issue persists.