Last night I've run into the same problem as you, after "upgrading" my old Dyn account to use "Updater Client Key" because it seemed a good idea to separate access account pass from more widely used updater key BUT THERE IS NO WAY BACK!
And... surprise! The web management page of my Comtrend router only accept keys up to 16 characters length, far less than needed for Dyn key.
This is the work around that solved my problem:
1) Insert the data of the Dyn updater key in the router as "XXXXXXXX"
2) Save/export the router config data using the router web page. It is a text file.
3) Edit the config file looking for "XXXXXXXX" and replacing it with the real Dyn updater key.
4) Save the modified file
5) Import the modified config file into the router using the router web page and reboot router.
I wish you the same luck as me.
Regards
Reinaldo