If you can't relocate, trying moving away from any current obstructions to the signal. I improved a friend's signal by moving from the floor below the desk up to shoulder height. You can have someone move the router around like you used to do with "rabbit ear" TV aerials to find any better locations.
Something I have not tried - you can get ac non-contact mains detectors that tell you if a switch or electrical device is powered (50VAC and up). They are $10-20. You can use them to detect wiring in standard walls. If you can keep the router away from house wiring, especially if between the router and most of the house, it may help.