I have tplink 941hp router and tplink 855 range extender.
My probldm is that the wireless connection between the RE and Router has delay and lag, when i ping the Router's IP through the range extender, i got time out and delay, i even put the RE next to the router got the same problem.
I connected the RE through wire to the router, it worked fine.
I use another generic range extender which works fine wirelessly.
I contacted the local support and they tested the device in their office and worked fine.
I did multiple factory reset and system upgrade with no benefit.
This photo example