When I connect with Mobile VPN with SSL, I can ping stuff in my network by IP address but I'm not able to resolve any DNS names.
Not sure where iOS comes from, but either way it sounds like your Firebox isn't providing an internal DNS server to clients.
Note that in both the Mobile VPN with SSL configuration, and in the Firebox global networking configuration, there are fields for DNS configuration. If you don't configure anything in the DNS settings for mobile VPN with SSL, it will use the DNS options in the global DNS configuration.
I would recommend you bookmark this help topic, it covers this and a lot more in detail: https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/mvpn/ssl/mvpn_ssl_tshoot_c.html