Thanks for your help, but I found another way to get it all working without need to SSH into the console.
All you need is luci-app-aria2 plugin installed (and ariaNG plugin)
Then go back into the router config, going to More Settings > Advanced > then log in.
You’ll see a new tab called services in the luci web gui, with Aria2 Settings in the dropdown.
All you need to do is select the enable checkbox and set a download directory. Then use webui-aria2 or ariaNG to access the web gui of aria.