rodi repair. fitting stuck

I am building a new tank and moved my RODI unit from my laundry room to under my kitchen sink. My tank is directly on the other side of the wall of the kitchen that the sinks sits on, so I and plumbed the unit directly to the ATO reservoir. It has 2 Optical sensors, a solenoid valve, and a...

I need to swap out a part on my RODI Unit. I can do the repair - but the fitting is STUCK hard. It’s a John Guest fitting. I know to push and hold the little ring down while you pull out the tubing. Problem is this ring is pulled out and will NOT push down. I have tried a screw driver but I...