While you're waiting for a replacement router there's a couple of things you could try.
On the back of the Unit there are four external antenna ports, next to this there's a small black switch, make sure this switch is firmly selected to Internal and not External or halfway in between.
Also look at the individual antenna ports are they in good condition, fixed permanent and not loose or broken, these external antenna ports are directly connected to the internal antennas via a balance line strip feed and any damage to the PCB area can adversely effect the internal antenna performance.
