I would phone contact, either sales or tech support and ask if they have any stock or send out replacement antenna.
If they don't you, could see if places like eBay, Amazon or NewEgg have some replacements or some 3rd party Mfr antenna.
If you can find anything there, you could check out these guys:
http://hawkingtech.com/products/wireless/antennas_and_boosters/hai7sip.htmlClick on the How It Works to see a picture of it. Others have connected these to DLink routers.
Lastly, if nothing above seems to work for you, you could just turn off the radio on the 615 and connect one of these as the Wireless radio:
D-Link DAP 1360, 1513, 1522, 1525, 1533You would get the benefit of upgrading the WiFi to using both 2.4 and 5Ghz radios for better flexibility and operation.
Let us know how it goes.