Using OpenWrt, IMO, is mostly advantageous over DD-WRT.
-
OpenWrt is in active development, so the linux underpinnings (including the Kernel) are generally up-to-date and updated versions get folded into the development branches (which eventually become a stable release). DD-WRT, on the other hand, is rather old and hasn't seen significant changes in many years
-
OpenWrt is very modular and extensible. There are tons of packages available in the repository, allowing you to run a large variety of services and tools, including modern VPN options like WireGuard and such.
-
DD-WRT has a few different build 'versions' (small, big, mega, etc.) that have functionality baked-in. You can't add to or remove those core features to customize your installation. OpenWrt starts as a base image and you install what you want/need (in your case, 3G/4G dongle drivers and such, etc.). You can also build your own custom images, if you want.
-
I can't really speak to DD-WRT's current security status, but I will say that OpenWrt rolls the latest security patches into the maintenance releases regularly, so it is up-to-date with security.
-
The interface for OpenWrt (LuCI) is much more modern, more user friendly, and extensible, whereas DD-WRT is rather old. It is also pretty straightforward to configure the router via ssh sessions where you can issue UCI commands and/or directly edit the config files.
And there are plenty of additional things I prefer about OpenWrt. It is not to say that DD-WRT isn't a good system -- it has some great features and actually works better for certain wireless chipsets (due to the closed-source nature of the wifi drivers for those chips), but for everything else, OpenWrt is great. YMMV, of course. But since I started using OpenWrt ~5 years ago, I haven't looked back.