Samyang 135mm f/2 owners ... Help please!

I've had my lens for a couple months already, but I'm struggling with long exposures and having the lens either fully open at f/2, or stopped down to f/2.8. The photo's that come out are just pure white and are so overexposed

I've had success with getting 1x minute exposures at f/2.8 but only with a low ISO like 400. Is there any way I can take advantage of the fast aperture, whilst still being able to take 2-3 even 4 minute exposures? Anything past 1 minute is usually way too blown out even at a low ISO

Anyone's advice on this would be great