how old is the entire unit? Resin lasts about 12 years.

You can get a new unit with a digital fleck valve for 600

What does resin have to do with the brine tank not filling? After filling the tank yesterday it's still full so I'll have to wait to see if it's still not working. $600 is way beyond our means being retired on a small fixed income.

If you look closely at your photo (I've inserted a close-up), you'll notice that small "hatch" with two hex screws. Bypass the water, and carefully undo the screws. Inside you should see a small wire mesh tube and two small nozzles, which will unscrew with a standard screwdriver. Be careful, the parts are small, plastic and very easily damaged!

Remove all three parts and clean everything thoroughly. I often soak them in a small amount of CLR or similar for a short while. Then carefully reassemble (note that the parts only go in one way correctly, don't force). Open the water valve and check for leaks.

That may help.

The question about the age of the unit and the resin is getting at if it makes sense to replace the entire unit.

You replaced the control valve with a used one. You have no idea if it's good or not (my guess is it's not, since it's not working).

You can try to rebuild that control valve. You can try to replace the valve again. Or - depending on the age of the equipment, you can replace the unit.

The other choice is you can bypass it completely.

Seems to me those are your choices.