I believe after the reset or the Power-on Reset like @Beaker mentioned you’d have to go through the initial setup wizard to get it on the Internet again and then added back to your site:
https://help.alta.inc/hc/en-us/articles/33805681985435-Route10-Initial-Setup
As long as it hasn’t been deleted from your site then it should pick up it’s configuration from the site once it’s added back in.
Otherwise I think SSH into the router would have been the only other way to adjust the WAN settings before it had been reset. To quote @Alta-Jeff from a similar topic “…but if you are savvy with ssh, you can edit the /etc/config/network file manually, update the IP address there, and run /etc/init.d/network reload, and be back online without a reset.”