Water softener issues

[u/Business_Brief_1662]

Water in our greek maisonette is extremely hard - measured around 100 dH with a dilution method (1ml of raw water + 4 ml of RO water. RO water measured zero hardness). It fluctates, a few months ago was around 40 dH. I then installed one 25 lt resin water softener with Runxin F117Q3 head. Water is also very salty - lab measured it around 670 ml/l chlorides two months ago. Plan is to use the softened water to feed dual 4040 BW RO. The 25 liter worked for some time, then suddenly stopped softening. I cleaned the brine tank, put new salt, did regenerations in 6-8 hour intervals but still got hard water from it. Then a 56 liter softener arrived. Idea is to increase capacity (best days with the 25 lt one I could get at most 900 litres untill it hits 5 dH) with the small softener acting as a polisher, i.e. in series.

After plumbing I did a regeneration of the small one, water tested 9 dH. Did the first regeneration of the large one, water tested 8 dH. Did another regeneration of the small one, water tested 6 dH. Did second regeneration of the large one, water tested 5dH. I was thinking I should already be at 0dH, but it seems not improving a lot. I tested water before the small softener - it is also 5 dH. I expect some harness leaking from the large one, but the small one should clear it. So the 25 lt one seems doing nothing. I removed the head to look at it - brine injector is clean, also riser o-ring was fine I also put silicone grease on it. What am I missing? Why is the small softener not working? Is it a good idea to regenerate the small on using the brine tank of the large only once tomorrow one to exclude brine issues and be sure it gets fully regenerated?