Worked it out, the link that works is http://192.168.8.1/webui-aria2/index.html
Next problem I had when logging into the webui-aria2 interface I kept getting the following error
Oh Snap! Could not connect to the aria2 RPC server. Will retry in 10 secs. You might want to check the connection settings by going to Settings > Connection Settings
I solved it by first formatting a USB stick as exFAT, naming the stick USBexFAT64G and inserting into the USB port on my GL iNet 300M
Then going back into the router config, going to More Settings > Advanced > then loggging in.
Once in the Advanced config, go to System > Startup
Go to the very bottom of the page called Local Startup, place the cursor on the line above where it says it exit 0
Then paste the following:
aria2c --enable-rpc --rpc-listen-all --allow-overwrite=true --auto-file-renaming=false --continue=true --dir=/mnt/USBexFAT64G/download --max-connection-per-server=4 --max-concurrent-downloads=1 --max-resume-failure-tries=10 --daemon
Click submit and reboot the router.