You need to move that air. Don't worry too much about the ambient temperature, i know units working in tropical climate at 40°C just fine.
You have to increase the volume of air you are moving. If your miners are inside a room or warehouse, you need to move the air that goes in and out of that place. If you also put them inside a container, the container has to move that same amount as well.

You can do as NotFuzzyWarm says and guestimate 250CFM per unit. I'm not really sure about the latest models like the T15, but something like 3000CFM would do. Find something that can move that, such as a one giant fan, or two 1500CFM ones, 4 750CFM ones, etc. MOVE THAT AIR OUT!



If you have seen mining containers you will notice they basically replace the entire wall on the long side so it can flow lots of air, that works great outdoors, but if the container is inside a building, you must make sure that building is letting move that amount of air as well.



Yes i think there are inline fans strong enough, but the smaller the fans the noiser. Also you are fighting the duct itself, you have to measure the cfm in and out of the ducts. For the time being you could try replacing your inline fan with something stronger...

MV 250 provides 910m3/h = 535 cf/m, this is too weak. You can replace it with the MV EC 250 but this only gives you  971 CF/M, so you'd need at least two more ducts as a minimum. (or larger diameter ducts, which move more air).