Summary

Print with 20% infill.
Super glue parts 1 and 2 together as per the diagram to create the base.
Screw the base to the wall, place the router onto it, and then measure and mark the required location of Part 3.
Remove the router, screw Part 3 to the wall. Then re-place the router onto the base and use Part 3 to clip the router securely into place.

Part 3 is needed so that the router does not get knocked off the base, pulled off by the cables, and is more securely attached.