When UPnP is enabled I cannot log into Cisco DPC3939B

We have the comcast supplied modem/router Cisco DPC3939B. I am trying to setup cloud service on a WD My Cloud EX2 Ultra NAS(network attached storage). When I enable UPnP on the Cisco and have the WD My Cloud plugged into the Cisco I cannot log in to the Cisco. If I turn off the wd and power cycle the cisco then I can log in again.  If UPnP is turned off on the Cisco and the wd is plugged in I can log onto the cisco but cannot access the wd from outside my local network. Any Ideas? Also I have read that you really should disable UPnP for security reasons but I am at a loss as how to get the WD cloud to work without it.