Again, I have no expertise in DynDNS. But as I understand it, it is used to find a way from Internet into your local network. Therefore, an IP address is needed. DynDNS is needed if you have a dynamic IP address (given by your ISP). DoT doesn’t change your IP address (therefore, you would need a VPN service).
If you need access from Internet into your local network, you need DynDNS. If not, not. 
This is what you need.
Alternatively, you could play with your blocklist settings.
You could manually set a DNS server (should be your Fritz!Box IP address) in the iPhone network settings for your home Wi-Fi.