You can look into a VPN service w/ fixed IP (such as the link I gave above) then with some port forwarding etc. etc. on the router you'll probably be able to access the SonicWall WebUI remotely in a similar fashion to what you're expecting with DynDNS.
Also, the Teltonika RMS service has some VPN features built-in, but I've not used it. There might be a way to tunnel through to your SonicWall WebUI from Teltonika RMS VPN but I have no idea how / if that's possible.
Alternatively, can you admin SonicWall via ssh on your LAN? If ssh is an option, Teltonika RMS has a Remote CLI (alongside the Remote WebUI). From the remote router CLI, you can then ssh to anything on your LAN. The reason I mention this is because Teltonika does a '10 year' RMS connection pack, for one router, for approx. EUR50 ~ish ... and you're never going to find 10 years of VPN + fixed IP that cheaply.