Cannot access OS X via LAN after mirroring onto different machine

Hey :)

I had this 2011 Mac mini Server that I used as an always-on machine, with El Capitan and Backblaze installed on it, backing up all my stuff.

It's given up on life, so I thought I'd mirror its disk onto a 2014 mini that I had lying around, so that I could resume its operation there. Mainly because reinstalling requires me to also reinstall Backblaze and "inheriting" the backup from the old machine is a major and time-consuming task, and also because hey — if it works, it's easier.

It does work, except... I can't see the machine on the network. I can't mount any shared folders, I can't ssh in, I can't access screen sharing, and I can't ping it. It itself can access the interwebs just fine, it's just as if it's got on a permanent firewall that I cannot get through.

I found this thread, https://discussions.apple.com/thread/2554739, and using tcpdump as suggested there to monitor the activity on the network adapter, I can see my attempts being registered — whether it's ssh or ping, I get log output that at least the attempts reach the NIC. They're just rejected.

I've tried enabling the firewall, disabling it again, and turning on and off sharing services to no avail. I noticed one thing, though, and that is that after mirroring OS X onto the machine and booting in, the newer mini still kept its network name from the installation that was on it beforehand. That is, the network name from the dead mini, let's call it "mini2011.local," wasn't mirrored over. Instead, it had the previous network name from the "new" mini, let's call it "mini2014.local."

This got me thinking that some of the system settings must be stored in NVRAM or SMC, or somewhere else that isn't on the disk itself. That explains why a Mac can access internet recovery via Wi-Fi without me entering a password, once I've used a Wi-Fi network inside macOS previously. So I've tried resetting SMC and NVRAM, but still to no avail.

Next up, I'm gonna try reinstalling El Capitan on top of itself, and see if it does anything. Currently downloading the installer.

But, I'm really curious if any one of you has any insight into this. If it makes sense to any of you? It'd be nice to be able to "fix" this without having to reinstall from scratch.

Oh, another factoid: The 2011 mini booted with a grey screen and apple, while the 2014 mini at some point was upgraded to whichever version introduced the black boot screen with a white apple on it, and it retains this boot. I believe this was introduced with Mojave? Either way, AFAIU this bit of code is stored in some EPROM (probably NVRAM or SMC? are there more places?), and I'm wondering if there may be some mismatch here with El Capitan not playing ball well with some updated ROM code...

And yeah, in case you hadn't noticed: I don't care much for upgrading, and won't, at least past Sierra. High Sierra and above are garbage IMHO, and if it weren't for massive software incompatibilities, I'd stick with Mavericks or Yosemite.