Apartments are small and easy to cover.
Just get a more powerful router to start with. Maybe one with swappable antenna jacks.
Position the antenna perpendicular to the direction you want to aim them. Maybe replace one antenna with a flat panel directional antenna aimed where you need coverage fill.
If that doesn't work, you'll want to run ethernet (apartment, so tuck under baseboards) to the dead zone and install a wireless access point (WAP) at the end.
only ~900?
Not bad.
Typically see quotes in the 4-figure ranges.
You have to remember if you aren't DIY you've gotta pay for their labor, knowledge, and overhead.