I want to seal the mortar on the entrance way of an apartment from the 1930s

[thesurfer]

I have been renovating the living room and bedroom. That has gone fine.

However, the entrance way (see picture) has lost contact between the interior mortar and the inside wallcovering that is attached to it. As the exposed bricks and their mortar show no moisture, I am curious why that old wallcovering is coming off. I can easily use 'Husfix' to fill the hole. However, I want the correct sealant for that area as I will remove the old paper and install Duro EasyCover.

I don’t want to have to come back in a couple of years to fix it again