I'm pleased to report that it truly was as simple as installing Pi-Hole, configuring the Pi with a static IP, turning DHCP on the Smart Modem off and activating DHCP in the Pi-Hole's configuration menu.
To give the best speed between the router and the Pi as possible I used one of the ethernet ports, and if it has resulted in a slow down, it's hardly noticeable.
I can even confirm it even works when the 4G backup is active - had a drop out long enough recently for the backup to kick in and it continued to work without missing a beat.
Pi-Hole does have it's limitations, like it won't block ads on Youtube before/during/after videos, but the web has been a much cleaner experience since turning it on. It even works on in app ads on my Android tablet.