Yes, it is quite a long and twisted thread, but I feel like it needed to be. Trying to trouble shoot this problem across two forums would have been huge, and its been a massive help finding people who are fluent in both areas I'm dealing with, so thanks again!

@whale-av I still don't understand why mac os would be ok with one router, and not another. That feels weird to me.

I got the WRT54GL v1.1. I've tried re-booting the mac with linksys as my preferred network, but its doesn't make a difference. I still get 'Network is unreachable (51) coming up.

I'll send you my email, very happy for the network help if its on offer! Its just not behaving, or rather misbehaving, in a way that makes sense to me right now.

@toddak Hello Toddak.......
I will post this here first. Error 51 is (for a mac).........
"51 ENETUNREACH Network is unreachable. A socket operation was attempted to an unreachable network."
So I will try to get the same error in my OSX partition, but it might take a while!.
Googling that error brings up a lot of chat about VPN's...........? are you running one on your home-to-work network?
Are you using "udp" for "netsend" (the -u -b switches I think)?
Do you get the message if you open just Pd........ before you open a patch?
Are you sure you are connected? Check by "pinging" a Pi? Check by pinging the router?
If you link (with a network cable) the "wan" (single rj45 socket) of the Linksys to your home network does that fix the problem.
Has the mac the correct information in the "prefs" netwok page..... Ip, subnet. Gateway that match the router info?
I will email you now so that you can send me as much as you have already found out........, and when I get a chance I will email you a zip of some screenshots of "good" router setup pages for your project....
David.

@whale-av Finally, something about networking just clicked and I think I solved my issue. Posting here as it resolves some of the thread and might help others.

When setting up the second pi, it was connected to the network that will be used in the field, not the home network. When I was looking at all the addresses for Broadcast, network, etc, they all looked substantially different, and I remembered an earlier comment in this thread about issues using different subnets.

Bam. Lightbulb moment. It wasn't working because all the broadcast, network, etc setting were found when connected to the home network, not the 'field' network. When I set it up using the setting on the field router, and connected my laptop to this network too, all was fine.

There still might be an issue in getting it to connect automatically to a network, but we'll save that for email when, and if, it happens.

@toddak

Wow, just read through this whole thread, 4 yrs later. I relate to all the obstacles as I'm too building something that requires networking. When you think you've finally got it right, something else comes up hehe. I'm curious though, how did your project turn out?? Are there any photos/videos/recordings of your show?
Best regards // Anton