@AlexOs 

Good to know you have tried some tips RE unstable troubleshooting guide. Thank you for staring a new thread and providing some case details here, please also check if you tried below suggestions: 

1. To help isolate the issue, it is recommended to set up use different Wifi SSIDs for 2.4Ghz & 5Ghz  on router 

. At the location of range extender, please make sure network devices can get stable connect when connect to 2.4Ghz network of router 

2. With Smart Connect OFF,  try a different 2.4Ghz Wi-Fi channel on the router:

Scan for nearly Wi-Fi channels using Wi-Fi analyzer app or software and select a less crowded channel for router.

At the same time can you help confirm some case details here for some further investigation: 

1. The detailed network topology, e.g.  Archer C6  <wireless> RE300

What are the devices that connected to RE300. e.g. Wi-Fi network devices that connected to RE dell Win10 laptop . 3 smartphones etc's
2. When did you buy the extender, and how long has it been working fine with the same router?

3. Does Range extender use the same Wifi SSID as router or different SSID, thank you 

4. How many network device were connected to extender 5Ghz wifi band before it left only 1 device on 5Ghz?

Will the 2.4Ghz led will come back on the extender or if 2.4Ghz will reconnect Status page after reboot? 

Hello   @AlexOs 

Thank you very much for case details, TP-Link devices usually works on channel 1-11, you may select a channel among 1-11 

To help isolate the issue, can you unplug RE300 and test if router's 2.4Ghz is stable for a 1-2 days. (As this router worked for 3 years, i suspect if here is something wrong  with router)

Or do you have a different AP or router, you could test RE300 with it as well. 

At the same time, we’d to create a support email TKID220808529 to assist further, please check your email inbox later. 

  @AlexOs Glad to know the problem got fixed, thank you sharing solution here.