Thanks very much!

Since this post, I tested for phosphates, they were high, so I had those taken out with Phosfree. I then added 2KG of powerchlor to shock the pool, as recommended by local pool store.

Warnings went away, chlorine level came up.

I set chlorinator to 60%, within one week chlorine was gone out of the pool... with warnings back on.

I assume the issue is a new cell is needed