Yes. Second hand DIR-868L is the way to go if you want inexpensive AiMesh setup. But then, it just the same like buying a second hand car. Lucky or Unlucky. 

By the way, I think you can you use the Optimise function in AiMesh settings or Reboot in System settings.

Both can be done in the ASUS App from your phone.

Either way should give what you want. Though, Reboot will reboot AiMesh main router and nodes.

Regards

Same situation here and i did some digging and found the solution.2.4GHz wifi Channel bandwidth 20MHz and Control Channel 6. If not working try change to channel 1 or 11 depends on wifi interference around you.