I tried to post this but it did not make it: so again, another option is to flash dd-wrt onto your router. Look at this link: http://www.dd-wrt.com/wiki/index.php/Netgear_WNDR3700
That said, be sure that you are comfortable with the flash process. Read the instructions CAREFULLY, and print them out first, if need be.
I recently purchaced a linksys ea4600 router to replace an old dlink 524, and it too limits me to one ddns service, therefore I intend to put ddwrt on it, since ddwrt allows multiple ddns servies, and you can also do a custom one. I don't know why companies do that sort of thing. Perhaps there is some economic advantage?
BTW, I have a couple of other older routers with ddwrt, and have been working well for over 8 years.
Regards,
Bob