You are probably giving them attack orders(attack path arrows are red, move path arrows are blue) , which means they move closer until they are in attack range. On level 6 motorized infantry gains range, so to attack they don't have to be on top of the city. That should work if you juts click on the green move button, which appears if you click on a stack. If you still have problems, check if you meet all these requirements(from an older post from me ( forum.conflictnations.com/inde…-occupying-new-territory/ )):


If you want to occupy something, you need stuff that can occupy in your
stack and have this stack on the province/city center once without
attacking something in the center (after attacked unit is dead or
retreets your can conquer, but not while fighting) or having right of
way with the current owner. If you want to know if something can
conquer, click on the unit type (e.g. Combat Recon Vehicle), than search
for a "can occupy" (Combat Recon Vehicle does not have one).
Basically every infantry (unit from the infantry tab in research) can conquer except special forces. Tanks are not infantry.


If a stack contains one ranged unit (like towed artillery or level 6+
motorized infantry) and you set it to "attack", it'll move closer to the
attacked thing until it's in range to shoot said thing and will then
stop moving.