A new (or renovated) apartment can be that much insulated that the signal is blocked.
Do you have network outside your apartment?
Do you have telephone coverage in the apartment and how is the signal? Weak? Strong?

There is no strong signal on the back side of the apartment as well.

If I go out (front side of the apartment) the signal is strong.

Not tried, not succeeded restart your phone, sometimes this may help.

If not, is it only your phone that has no connection? When you have a visitor, that visitor also?
Does it improve when you put the SIM in another phone? This test can point out if it’s your phone or your sim that is the culprit. Or the building of course, the construction may block all signals. I suspect the latter being the case, it is a rather new apartment, I guess?

Proximus did offer a Mobile Coverage Extender in the past, but as far as I know, not anymore. I may be wrong though.
Do you not have the possibility to use wifi instead indoors? This would solve your problem easily.

Maybe some else drops in with a (real) solution?

FYI: you may be able to use vowifi (voice over WiFi) at home on your smartphone, I have to do that here in our apartment also, due to poor mobile reception.

see → call via wifi