R6260 ROUTER WON'T SHOW WIRED COMPUTER

> [...] our old 1980's unix based DISC server [...]

   Not a very detailed decription of anything.  Is there a console
terminal on this mystery machine where you can log into it without its
being on the LAN?  If you can talk to the thing, then output from
commands like "uname -a" and "ifconfig -a" would be interesting.

> [...] If I plug in the old Asus router the DISC server pops right up
> under connected devices. [...]

   And when it does, what do you see?  Copy+paste is your friend.  Do
these two routers use the same LAN IP subnet ("192.168.0.*", or
whatever) for their IP addresses?

> [...] On all of our older routers we manually assigned the DISC server
> an ip address under the router settings so it would always stay the
> same when the dhcp lease time expired. I don't know how to do this if
> I can't see it under the router attached devices. [...]

   I know almost exactly nothing about your mystery machine, but...

   I would not expect a seriously old UNIX(-like) system to use DHCP to
get its IP parameters.  I'd expect it to be configured with static IP
parameters (independent of the router and its reserved dynamic
addresses).  In such a case, that system might not try to communicate
with the router -- certainly not for DHCP -- so the router might not
notice it.  (And if it's configured for a different IP subnet, then it
would be pretty thoroughly unable to communicate with the router.)

> [...] I have tried multiple ports on the router and in the 16 port
> switch and it still will not show up. [...]

   If it doesn't work somewhere, then it shouldn't work anywhere.

> [...] I know the cable works correctly because everything functions
> properly when the old router is plugged back in. [...]

   A better clue would be a positive indication on a port-status LED
indicator on the router (or network switch) where the cable is
connected.  Even better if there's a similar indicator on the computer
itself, but a very old system might not have one.  But I'd bet more on a
bad (inappropriate) configuration than on bad hardware.