So, this question is more how does NAT function. There are different NAT configurations, but basically when you connect to anything behind a router, that router maps a port to be used for the request. Traffic matching its destination on return is then compared against an internal table and sent back through to your device. Opening these ports do not directly increase speed, but they do allow you to join DHT/PeX swarms. If you see an increase in speed its because you are effectively being saturated by connections passively through the swarm.
In a normal situation to connect to these swarms, you would either need to open a port pointing to the port number you configured your torrent application to use thereby making you visible to it, or enable UPnP which dynamically maps ports for the connections to work. Typically, you wouldn’t want to enable UPnP as it is then possible to externally query the router and pull a manifest of UPnP advertised devices that exist on your internal network, however.
The problem with opening ports in your router if on a mobile network is that most networks use CGNAT. This is where your router does not hold a publicly routable IP address on the WAN side and instead maps out a single public IP with many (possibly thousands) of other devices. In this case, you would need something like a VPN service that supports port forwards. They would give you a port that they are forwarding for you. You would take that port number and from the device you connected to the VPN from (like your PC) enter that port number into your torrent client’s “listening” port field.