For future reference, the Tomato GUI expects to have values input into the fields, but some fields come with no values by default. HTTP_WANPORT must have a value (such as 8080), even if remote access is disabled.

But this should not have anything to do with inability to connect to the Internet.

A further note regarding the HTTP_WANPORT Error:

As FRiC pointed out, a value is required for this field. In my case, I did not want to enable remote access right now, I just wanted to change my password. Because I had not explored the remote access function, the field to input a port was not available to me. It only appears once you change it to HTTP or HTTPS. So, I changed it to HTTPS, put in 443 (I don't run any sites locally, so no conflicts for me). I changed the default password and hit save. Once that is done, I changed it back to Disable and hit save. This worked fine.

Hope that helps someone.

Also, as FRiC pointed out, this has no bearing on connectivity on the WAN port of the router, in my experience.