My approach would be to chip away any of the remaining substrate that might be loose, scrape it, pick it off, whatever. The goal would be to leave very solid material exposed.

I don’t know the equivalent in Europe where you are, but in the US we have Tapcon screws. They come with a masonry drill bit that you would use to hammer-drill several holes along the exposed material. Then screw in the screws until the head of the screws protrudes an appropriate amount to leave room for whatever is needed above the screw head.

These screws now act as a little rebars to support what you build on top. Looks to me like you might need concrete adhesive, little concrete nub mushed around the screw heads, thinset, then tile, then grout.