You say you cannot relocate the router. But standing it horizontal of vertical, and vice versa, can make a big difference. As can moving it only half a meter in any direction.
Adding a second unit is (obviously) around the same cost as replacing the original unit with something more modern: I just swapped out a perfectly functional 54 Mbps thing that covered an apartment nicely, with something cheaper that seems to be supplying half the community with channel-bundled 866 Mbps.
Also, the easiest fix is one actually the hardest to do without the right gear or farting around: find a less noisy channel, if your router doesn't do that automatically from time to time.
Nirsoft may do the job for you, though.
Xirrus too.