Before buying new equipment you can try making some adjustments.

First, get the wireless access point (the ISP router) up high. Put it on a high shelf.

If the router has external antennas, make sure all of them are pointing straight up and parallel to each other. If the antennas are internal, leave the device in the orientation for which it was designed - if it’s made to stand on end like a book don’t lay it down.

If you have access to the configuration page see if you can set the wireless channels to auto, and the channel width to 40 MHz for the 5 GHz band and 20 MHz for the 2.4 GHz band. You can even try 20 MHz on both. Those smaller channel widths will give lower bandwidth but improve coverage due to hitting less interference. (Since you said a flat, I’m assuming lots of other residents with their own devices so we want to avoid interference.)

If auto channel isn’t an option, get an app like Wifi Analyzer and see what channels have the fewest other networks on them and set the channels to one of those.

Do you have 5g home service available? One “landline” ISP and one wireless ISP is a good way to get backup.