I've been trying for a few days. I used to be able to access this. I have also rebooted the router from the router's page. Still the same problem.
Here's the page I get when I try to go to Transmission GUI (I don't know what to do with this, as I am not a Tomato expert):
Unauthorized IP Address.
Either disable the IP address whitelist or add your address to it.
If you're editing settings.json, see the 'rpc-whitelist' and 'rpc-whitelist-enabled' entries.
If you're still using ACLs, use a whitelist instead. See the transmission-daemon manpage for details.