I have a WiFi setup where i am using TP-link TL-WR845N as my main router and then using another Mercusys MW302R router on range extender mode to extend the reach of internet.

Issue : 
Everything works fine on the main router but often times i get "Connected without Internet" issue on my extender router. I need to hard-reset my extender router to fix the issue. Even after Hard-Reseting, it work fine for 2-3 days and then i get same "Connected without Internet". Is there a permanent fix for it ? then buying a different barnd router ?

I have tried contacting TP-Link support multiple of times but they are so rude and restless, that you can not deal with them thats why i am putting it here.