The simple answer is that communications protocols and frequency bands have changed as wireless modems have evolved to provide faster connections. Older PCs will still work, but the data transfer speeds will be very slow by comparison to more up to date kit.
Older wireless systems using 2.4 GHz frequencies have a bit more range and signal penetration than the current 5 GHz systems. So you get blisteringly fast transfer speeds with the newest kit while relatively near the modem, but equipment further away that previously worked fine, may simply be unable to connect.
There are various range extenders and antennas available to help in these situations and sometimes simply resiting or elevating the modem higher works too. Mine is sited on top of the kitchen cupboards and on a good day I can get a connection in the neighbouring street.