I'm also a little unsure of what your saying?

But if you are filling the gun with the cylinder connected, then both gauges should be working if everything is correct.

If your guns gauge is not working then perhaps that is broken, if the dive cylinder gauge is not working then perhaps that one is broken, but it's very unlikely that both would have failed.

If the cylinder gauge is working while your filling, but then goes to zero when you stop, then your connection whip is leaking, as it's not holding air between the cylinder and the whip.

If this is the case, it may be that you have put some air into the gun - but not enough to register on the guns gauge, and the pressure you have seen on the whip is not real, as its pouring out somewhere, so giving a false reading.

I'd initially make sure the whip was working properly by using a blanking plug or another gun to check it. If that is all good, then check the foster fitting to the Daystate to make sure it's connecting properly and not leaking. If that is also good, then slowly fill your gun and the cylinder gauge should reflect this, the guns gauge should also reflect this, but perhaps be a few bar behind as the pressure in your cylinder is "pushing" up the pressure in your gun.

Like all things, if you have someone local they could get to the bottom of this is seconds if they know what they are doing, it may be worth a shout out if you can't get is yourself

James