If you have a ping response from the printer then it should be possible to print to it.
However I would much prefer to see the printer is picking up an IP automatically.
It is a very unusual IP..
Having the IP in a different gateway can work but is unusual.
What IP is the Express?
How is the express connected to the rest of the network?
Does the HP 2600N have drivers for 10.10.5 (or at least Yosemite??)
Manually setting up printers is not something I have done with Macs.. it was a nice thing with windows that you can do it.
I might need to look up details of this.. I presume the printer is postscript so printing to it should be possible even without the latest drivers if it comes to last resort.
I do however need to know the full network layout.. and I need to see how the domains are working.
The best idea here is to do a netstat check from the computer via the airport utility. Post the results for me..
I want you to be wirelessly connected to the express when you do it. (you do this from network utility which is hidden in Yosemite.. just search for it in finder).
Routing tables
Internet:
Destination Gateway Flags Refs Use Netif Expire
default 192.168.2.254 UGSc 33 2 en1
default 192.168.2.254 UGScI 0 0 en0
127 localhost UCS 0 0 lo0
localhost localhost UH 4 3310 lo0
169.254 link#5 UCS 3 0 en1
rays-airport-extre 5c:96:9d:67:53:cd UHLSWi 1 53 en1 139
tcgen1test.local 0:1f:f3:c7:c1:a6 UHLSWi 0 125 en1 115
aegen5router.local 70:73:cb:b3:71:a4 UHLSW 0 5 en1
192.168.2 link#5 UC 8 0 en1
192.168.2 link#4 UCSI 5 0 en0
192.168.2.65 0:1f:f3:c7:c1:a6 UHLWIi 0 0 en0 1145
192.168.2.65 0:1f:f3:c7:c1:a6 UHLWIi 1 928 en1 1150
192.168.2.67 0:11:41:30:14:bb UHLWIi 0 0 en0 948
192.168.2.67 0:11:41:30:14:bb UHLWIi 0 166 en1 953
192.168.2.77 d0:50:99:2f:6a:ad UHLWIi 0 192224 en1 532
192.168.2.169 localhost UHS 1 1 lo0
192.168.2.201 70:73:cb:b3:71:a4 UHLWIi 0 8 en1 989
192.168.2.205 localhost UHS 0 52 lo0
192.168.2.227 34:23:ba:f7:65:21 UHLWIi 1 11 en1 118
192.168.2.230 5c:96:9d:67:53:cd UHLWIi 1 0 en0 144
192.168.2.230 5c:96:9d:67:53:cd UHLWIi 1 0 en1 144
192.168.2.254 30:b5:c2:e2:fd:8c UHLWIi 0 1 en0 100
192.168.2.254 30:b5:c2:e2:fd:8c UHLWIir 34 15397 en1 1186
192.168.2.255 ff:ff:ff:ff:ff:ff UHLWbI 0 2 en0
192.168.2.255 ff:ff:ff:ff:ff:ff UHLWbI 0 2 en1