In order to achieve this goal, you'll need to know the port numbers and/or IP addresses that are used by the application. Discord uses ports 80 and 443 (443 is for text chat) as well as a random port between 50000 and 65535 for voice chat. However, because many applications also use ports 80 and 443, you can't route based on these ports. Additionally, you can't route based on random ports because you never know which port will be used for communication.
Unfortunately, Discord has many servers with different IPs, which makes it difficult to know which server will be used. However, if you can find out the IP addresses of the Discord servers you use, you can try creating a separate routing table.
To do this, go to Network > Routing > Advanced static routes and create an additional routing table. Choose the interface to be your mobile interface.
Then, create a routing rule for IPv4 (on the same advanced static routes page). In the rule, set the destination subnet to reflect the IP address(es) of your Discord server, and set the lookup table to your newly created routing table.
This way, traffic destined for your Discord servers will use a different routing table that has your mobile interface as its main WAN.
More information on routing can be found here.