Can download but can not upload. (No direct connections.)

[z0cMWWKmGfudsDG]

I knew this is an old question and problem, but it is still confusing me badly. I didn't find an exact and correct answer.
What a weird problem. I can download from others, but I can not upload for them.
My phones can use torrent client transfer between them, so my router seems has no problem.
I am using Linux "Mageia" 8, have tried version 4.3.3 and 4.4.4. Also tried another Linux distribution and three other Desktop Linux torrent clients. Same problem.
QBittorrent can find my client on phone, but has no any speed, whatever download from or upload to it. They say "Connection" as "BT", and say "Flags" as "H".

[slrslr]

Per my understanding one of the two peers have to be active one, meaning open port. I think that easier would be to open p0rt on Linux - you can find many tutorials on the internet for this.
If both peers are in same LAN, i think that direct/active connection should not be problem once you open port.
Else if peers are over internet, the router may be a barrier between the peers. Also there are tutorials on how to port forward on router (from your qbt file sharing port to your LAN computer IP - you set static IP to your LAN computer in your networking configuration - also tutorials available) or instead of forwarding on router, you can enable DMZ on router for your LAN computer IP, though that may decrease your security per what i have read - you would have to rely on your firewall and make sure your computer SW is up to date.
I am using VPN and that way i forward my qbt filesharing port on the VPN server to the VPN client internal IP using iptables firewall.

[z0cMWWKmGfudsDG]

Succeeded to connect the bittorrent client on my phone and transfer data by add IP address and port manually.
And, to make the icon green, just connect a lot of clients and transfer data. In fact, it can upload, just too less upload made me doubt. Also it can be done without enabling DMZ.

[slrslr]

@gaojicainiao Btw. if You are after transfer within home LAN peers, check qBittorrent settings, Bittorrent tab. There is option for local peer discovery. It may be related too. (not sure if is enabled by default and if needed to be turned on on both LAN peers). I guess it works without trackers and no need to turn on built in tracker option on Advanced config tab of the qbt settings.