Have you tried using the password recovery method and then setting the config register to 0x2142 and then see if you get any better response ? You should also open up the router, and confirm that all the flash, DRAM and if there are boot proms these as well. Check they are located correctly, and are not loose or anything. Give everything else in there a quick check to make sure it all looks to be connected OK.
Also remove any modules that are in either router as these may be causing problems.
Have you also tried the AUX port ? Does this give the same result ?
As these are second user that you have bought from somewhere (neither are current models), there is a possibility that they are faulty and that is why they were sold.