@MagNeto 

Hello, are there any Onemesh range extenders connecting to the C6 network? If not, please try disabling Onemesh feature on the C6, then reboot the router and reconnect to the wireless network to confirm.

What if you connect one of the devices to the C6 with cable, will you still suffer from the ping spikes? BTW, how many pings do you get by referring to ping spikes? You can also follow this article to understand better whether the ping is high:

https://community.tp-link.com/en/home/stories/detail/1392

@MagNeto 

Is that isp provided router+modem device acting as the DHCP gateway? If that is the case, I will suggest you configure the C6 in Access Point mode to avoid a double NAT issue, which may have an influence on the gaming. The following guide will tell you how to turn the C6 from Router mode to Access Point mode:

https://www.tp-link.com/support/faq/1384/

Meanwhile, we will also suggest you run a Wi-Fi analyzer on one of the wireless clients to review the wireless interference in the network, -78 dBm is not good, you need to move closer to the router, or you can try modifying the router wireless settings such as channel and channel width to see if any helps.