Since the difference is approximately 4 ms and is relatively limited, you may try the following checks to see whether it can be further reduced.
Could you please confirm whether both the WINDTRE modem’s QoS and Flint 3’s SQM using CAKE are currently enabled?
If both are enabled, please temporarily keep QoS enabled on the WINDTRE modem, disable SQM on Flint 3, and enable Network Acceleration on Flint 3 to see whether this helps reduce the latency.
If the WINDTRE modem supports bridge mode, you may consider configuring it in bridge mode and allowing Flint 3 to establish the Internet connection directly. This may reduce an additional layer of routing and NAT, avoid duplicated traffic processing, and allow Flint 3 to manage the connection and SQM directly.
Im NA West and my lobby ping is 11, in game ping ranges from 11 to 30 depending on the match, Epic is trying new things with routing so you won’t always have the lobby ping. They’re load balancing the matches across alternate data centers. I can ping the servers directly from my pc via cmd and trace route verifies this. One thing might help is using cloudflare DoH and forcing all clients to use it.
I know they're testing, but I've always had 30/40 ms and it's unplayable like this, only a week I managed to get 25 ms and it was completely another game
the windtre modem is the basic one so the various functions are crap, I think the only possible one is to put it in bridge but anyway I'm not sure if it's okay
I understand that but, it comes down to the connection you are using and your distance from the servers of the Game, if you can bridge the modem it should help.