@Maru86
Can we confirm who is your ISP, and what is the WAN connection type on the WR820N?
Will the internet LED on the router change to be orange when the internet disconnects? Please login to the router web interface page to check if there is a WAN IP address when the internet disconnects, send us the screenshot. If there is still a WAN IP address and gateway when the internet is off before you manually restart the router, please try to change the DNS server on the router to 8.8.8.8 and 8.8.4.4 under Advanced -> Network -> Internet, where you can find the Primary DNS and Secondary DNS, then monitor again.
Follow this guide to check more:
https://www.tp-link.com/support/faq/2237/