When I'm using the 88mm gun, it doesn't seem to have its full range; I can shoot at things but the shell detonates before it reaches the designated target.
I'm also using modular turrets. Could that affect it?

There could be a few possibilities:
1/ The tank shell hits an object on its path.
Tank cannon shells (88 mm) work the SAME as vanilla tank cannon shells. If they cannot penetrate the objects on its path, they will detonate on the blocking object instead of the designated target. It is vanilla behaviour, and my mod does not change it.
2/ You are aimining beyond its range.
Tank canon shells (88 mm) has range of 30 tiles. Yet the game does not prevent you to aim objects outside the range.
3/ Modified by other mods.
It is possible, where some mod(s) have changed the settings of my cannon shells. You may see by checking the item tooltip when hovering on the shell item. The blue line of text (under item name) shows what mods have made or modified the item.

I have not tried the mod you mentioned, so I am not sure about it.