So the problem was indeed with the NAS.
On Synology, in Network > General > Advanced settings, the "enable multiple gateways" was unchecked.
I had disabled this function because it was causing me problems with my Docker networks, which could no longer communicate with each other.
I ended up leaving this function disabled but added a static route to 192.168.20.0
I don't know if this is the right way to do it, but it works.
Anyway, thanks a lot for the help! I've learned more how to use tcpdump and other tools, and I'm very grateful :-)
On Synology, in Network > General > Advanced settings, the "enable multiple gateways" was unchecked.
I had disabled this function because it was causing me problems with my Docker networks, which could no longer communicate with each other.
I ended up leaving this function disabled but added a static route to 192.168.20.0
I don't know if this is the right way to do it, but it works.
Anyway, thanks a lot for the help! I've learned more how to use tcpdump and other tools, and I'm very grateful :-)