Inefficient transmission (need help)

[LonePheasantWarrior]

I deployed my private service via docker (the server is at home and the home device has a public IP).

When I access my own private snapdrop service via domain name, I find that it is very slow even if both devices are on the same LAN. After my observation, I found that even if the two devices are in different network environment (not in the same LAN), they can still discover and transfer data to each other, and the transfer rate is the same as in the same LAN.
That is to say, no matter how the network environment, the data always has to pass through my private server, which greatly affects the use experience, how to fix this?

The FQDN value in my /docker/fqdn.env file is set to my public domain name, is it not configured correctly here?

[LonePheasantWarrior]

Even if I change the FQDN to localhost, it doesn't help
current set: FQDN=localhost