I have 2 routers GL iNet and I am trying to turn my VPN as more anonymous as possible. I dont understand much about this, but I am testing in this website
I already tried to disable the "Allow Ping" in the router that is being used as a server, but I still see that is possible to detect that I am using a VPN tunnel.
Are you using a commercial VPN service? If so the IP ranges they use are well known and provide an easy way to check if someone is using a VPN.
Googling for further details about two way ping and VPN doesn't appear to bring back much info that doesn't reference the same test you've done. There's nothing that suggests it's a common test used by other parties to detect VPNs.
Or, to put it another way, it's probably having little to no impact on your anonymity. And 'solving' it would be unlikely to have any real world impact on whether your VPN usage is detected.
As it is, if you've disabled the ping on the router (and that's the VPN endpoint) there's not really much more to suggest. You'd need to find out more about exactly how the test works before it'd be possible to suggest anything else.
It appears you are using firmware that is not from the official OpenWrt project.
When using forks/offshoots/vendor-specific builds that are "based on OpenWrt", there may be many differences compared to the official versions (hosted by OpenWrt.org). Some of these customizations may fundamentally change the way that OpenWrt works. You might need help from people with specific/specialized knowledge about the firmware you are using, so it is possible that advice you get here may not be useful.
Ask for help from the maintainer(s) or user community of the specific firmware that you are using.
Provide the source code for the firmware so that users on this forum can understand how your firmware works (OpenWrt forum users are volunteers, so somebody might look at the code if they have time and are interested in your issue).
If you believe that this specific issue is common to generic/official OpenWrt and/or the maintainers of your build have indicated as such, please feel free to clarify.