I presume that you are using the LAN ports in back of the 655 and 645 for connection to the network and not using the WAN port right?
I would compare the settings of the 645 and the 655 and make sure the settings are correct. Shut off everything, SPI, WISH, WPS, all filters and such.
Can you get a DLNA connection if you turn off WiFi security on the 655?
Wonder if you should connect to the guest zone and test.
I can only presume that if the 645 is working and the 655 isn't in this configuration, either the configuration is wrong on the 655 or the 655 may not allow DLNA traffic has it doesn't support it natively. You may want to place a difference AP device in place of the 655 if this is the case and move the 655 to a lesser needed area or use it has a host router.
What FW is currently loaded on these routers?
You may want to take a look at these threads:
DLNA PostDLNA Info