Are you connecting to the router by cable or wirelessly to download? Does it happen different clients also?

Well, you found out the answer by yourself. The Unifi firmware doesn't support guest network feature while the latest firmware with guest network feature doesn't work for unifi. I would suggest to...

Find the detailed instruction here. C7 should be configured as an access point....

Devices which are 802.11n capable may not support 5GHz band. http://en.wikipedia.org/wiki/IEEE_802.11#802.11n 802.11n operates on both the 2.4 GHz and the lesser-used 5 GHz bands. Support for 5 GHz...

If the beta firmware doesn't fix it, why not re-open the ticket and tell them?

Wired connection to WPA2220 works OK? How about status of all lights on both units when you connect to internet or lose connection?