Try this: login to the forum using Safari on your iPhone.
On the phone: go to Settings->Safari->Advanced->Website Data. You should see an entry for concoursmustang.com. If you do, the cookie has been set and you should be able to close the browser and come back to the site without logging in again. If there's no entry, the cookie isn't being set.
Look at Settings->Safari. Is "Block All Cookies" enabled? If so, turn it off and try logging in again. Is JavaScript enabled? If not, turn it on and try again.