The PC that is not working (10.254.11.18) is not in the same IP range as the others on the right hand side's subnet (I'm assuming that the subnet is defined as 10.120.30.0/4).
If you really can't change that IP (which sounds rather odd), then you will need a subnet that includes that IP, which means your AS5505 router will need an IP in that range (e.g. 10.254.11.0/0), and associated routing table entries, and also your AS5520 will need to know to route that subnet to your AS5505. Your AS5505 could advertise that it handles that subnet range, but depending how complex the bits of your network you're not showing us are already, a static routing entry might be simpler.
Why can't you modify the 10.254.11.18 IP? How is it being assigned at present? I'm guessing that this involved its participation in some network structure you're not telling us about, which might substantially modify what I should be telling you about your routing issues.