Sorry but your going to have to bite the bullet on this one as has already been said, you will need to call in professionals.
By soaking the carpet in soda water etc you have made the situation worse which is why you have an ongoing problem.
The carpet will need cleaning first, to remove any soiling that is left , before uplifting the Underlay which will have to come out, the floor will have to be treated along with the back of the carpet, you will have to leave the floor exposed and carpet turned (not folded) back until dry before re-fitting fresh underlay and relaying the carpet.
If the carpet is not cleaned and treated first you will simply push the urine down into the new underlay and floor which has just been treated starting the cycle all over again.
Hope this is off some help.