I seem to be having exactly the same problem described above. I am using a RT-AC86U router and am the only client on the 5 GHz wifi, the other. clients are on 2.4Ghz.
I get get pretty regular ping spikes on the 5GHz wifi which make playing online games impossible. I know, for sure, that it is not my ISP as plugging in a LAN cable or using the already congested 2.4GHz channel works fine without any issues.
Here is an example of a ping test from my PC to the router IP (5GHz):
ping...
RT-AC86U Ping spikes on 5GHz Wifi
I get get pretty regular ping spikes on the 5GHz wifi which make playing online games impossible. I know, for sure, that it is not my ISP as plugging in a LAN cable or using the already congested 2.4GHz channel works fine without any issues.
Here is an example of a ping test from my PC to the router IP (5GHz):
ping...
RT-AC86U Ping spikes on 5GHz Wifi