go below disassemble the drain and replace the stopper unit. its part
f the drain unit.

hopefully you have easy access

Sadly, no. Concrete slab floor and what's more, on the other side of the
this lav is another one, a little larger and the sinks, toilet, and bathtub
are back to back with each other.

Just to be clear, this is not a pop-up stopper. It's a hidden plug that
drops down in the overflow passage way. Even with access I don't know if
there is anything to disassemble from below or behind.