For the dynamic dns concern in pfsense, it does not use the WAN NIC address but instead goes to http://checkip.dyndns.org in order to obtain the public IP. If you go to Services > Dynamic DNS > Check IP Services, you should be able to confirm this.
Also it is a bit weird to have a 20.x address, since that is technically a routable IP. However it looks like your Draytek may support bridge mode, something to try out: Configuring Vigor2760 for VDSL Bridge mode – DrayTek FAQ