I wouldn't recommend a phone. It would kill your battery, and speeds would be terrible. Find an older computer or a beat up laptop and install qbittorrent on it. You might also need to forward the port.

To allow multiple simultaneous connections over the network, computers use these things called ports, which are numbers used to identify the connection. When the operating system sees that a network packet has arrived, it looks at the port and then forwards the packet to the right application.

Routers create a local network that is isolated from the outside, and all the traffic that goes to or comes from the internet, goes through the router.

If someone wants to connect to you, they have to go through the router. By default, routers will just refuse any connection coming from the outside. They do allow connections from the inside going out. Note that after the connection is established, communication can be bidirectional. Think of it like a social network where you need to be friends with someone to chat. Establishing a connection is like sending a friend request.

Port forwarding basically means telling your router "if someone tries to connect to you on port XXXX, forward those connections to port YYYY on computer ZZZZ".