Tuning can't pump up my new walther reign UXT 25 cal

Air coming from the barrel means the poppet (valve stem) is not sealing. You confirmed you had it cocked which prevents the hammer from holding it open so apparently something else is preventing it from seating properly. Try dry firing it a couple of times to see if that manages to jostle the poppet back into a proper seated position. Then try again to fill it.

Leaking around the fill port is perhaps a little more complicated. Could be a combination of tolerance issues and how well the O-ring in the female Foster is able to conform to create the seal. Meanwhile you’re losing pressure as fast as you put it in so the O-ring doesn’t get the kick in the pants it needs to make it do its thing.