First of all i would like to note i can make it show up selectable in the Campaign menu, i understand the overlay system.

The problem i'm getting is that mods outright don't work/load in the campaign. These mods i've confirmed work in quick match and the lobby.
The mods i've tested so far is

The custom_campaign_example
Newa
Rares for sale
and multiple of my own creations.

Any help would be great. thanks

Mods most of the time need to be specifically adapted to work with the campaign script due to how the script declares resources to be used in a different way than the simpler quick match. Also there are often more things to consider in campaign than quick match, i.e. which resources are unlockable, which ones should not be available for the friendly faction, which ones should not be available in specific maps, and so on.

Also there have been some architectural changes on the script side almost each update lately, so even mods that were adapted for campaign usage likely need an update as well.

E.g. mods that add new factions (newa) need to be adapted for campaign so that the three factions present in the campaign are defined somewhere so that green/grey/brown resources are not used. Well, it is certainly possible to mod the campaign to support more than three factions, but that'll mean an effort in distributing the initial areas between the factions and setting up the stages so that multiple factions are present.

Mods that add/remove weapons or vehicles need to have them declared in an adapted invasion_resources.php to appear in game, or change the campaign resource loading to use some another way.

Alright, i understand this now. Thank you so much for your help