One thing is the question how do I set everything correct so that my units can call on map fire support like artillery or mortar by themselves?
It's a bit difficult to achieve, because you need the right chain of command to be present on the map. And since if you play with the mode where you control individual platoons on the operational map, they will often be scattered so that when you enter battle, the company HQ will not be there, or it will be the HQ of a different company, etc.
If you manage to get a platoon and its company HQ in the same battle, then they also need to have a communication link. In Mius, that usually means wire. If you are attacking, then you won't have wire comms as you enter the battle, and the HQ and its platoon will often be far away from each other. Too far for the wire layers (they have a max range of about 1 km if I remember correctly). So you need to move them closer together and then wait a long time while wire is laid. This can be difficult or impossible depending on battle conditions.
But OK, now you have your platoon in communications with its company HQ. If the company HQ has a mortar squad attached directly to it, this mortar should now respond to fire requests by the platoon. You need to toggle on the AI fire control for the mortar unit only. The icon is a human head with a gear inside. Toggle this off if you want to hold fire. Toggle "stay concealed" for the mortar to fire smoke.
The situation gets a bit more complicated if you want to use for example an infantry gun section that forms an individual 'platoon' unit of its own and is subordinate to the battalion commander. In that case, now you need two more units on the battlefield at the right time and right place, all located correctly and wired up - and since Mius Front automatically decides who participates in a certain battle and who doesn't, often one of these units gets cut off from entering the battle. This means you can't use the whole fire support system.
There's a game mode called "Battlegroups" where it doesn't split up units before battle, and this makes it much easier to use artillery. However, it also means very big battles and no options to strike at a weak point of the enemy line as you can with the other system.
I hope this helps.