No problem with C7 working as access point? I think it could be the WDS settings on the C7. Not that setting is wrong, since you can access some websites, and I guess the WDS status is "run".
Do you have the newest firmware?
Maybe it's compatibility issue? You know WDS with other brand router is not always successful.

So, working as access point, the internet works properly but the dns or channel (idk where is the problem) keeps disconnecting the internet, I'm unable to play a game for more than 2 minutes until it bug and disconnect again for more seconds.
I have the newest firmware and my objective is to connect to the main modem wireless. I tried and worked for 1 day but the other day it started to disconnect me like I said. When I'm on the DHCP range from the modem connected on the C7, the internet works until the DHCP from the C7 updates my ip.. And I can't define my ip to the range of the modem because C7 declares it's out of range.. I don't know if you understand, so, if I need to be more clear, I can manage some printscreens..

It seems you didn't disable DHCP server on the C7
https://www.tp-link.com/en/faq-1385.html