I'm new to photography and think I have a good grasp of the fundamentals of the exposure triangle. When using flash in a controlled studio environment, I want to set my ISO at 100 and compensate elsewhere for exposure.
I am using a Canon EOS RP Mirrorless. I'm using a trigger with speedlights on manual slave so I can test and control flash exposures light by light. The problem is when I set ISO to 100, the camera is exposing or metering for ambient light and shows constantly underexposed. Which constantly boosts by ISO to around 1600 and blows out every 3rd shot I take. I have to keep manually putting the camera back to 100 ISO, and this just continues. I can see that it is underexposing but I have tried every setting. Tried disabling AUTO ISO and FE Safety. I try changing my flash settings to manual but it doesn't let me. I think because when I have a trigger it defaults to group mode. I just cannot find an answer anywhere online.
I have my ISO Speed range from 100 - 12800. I set my ISO speed to 100 but it continually reverts to AUTO when metering. I am obviously doing something fundamentally wrong but the penny has not dropped. Essentially the camera is metering without seeing FEC. But I can't select Manual to change it, it only goes to Group Mode. Any help would be appreciated. I don't want to use any ISO higher than 100 because I want to understand.