Sorry, thought from the screen caps that the issue was with DHCP list. I've never bothered with MAC filtering so didn't realize there was another similar looking page and my limited Russian doesn't help either. :o
The screen caps are from ASUS stock and not Merlin as the IP address column is missing. Please can you check what firmware you are running.
I can't duplicate the error on an AC68 using beta 2 and chrome browser.
I second recommendation to remove spaces as they are not valid in hostnames...
As an exercise in using git, compiling and generally tinkering with the firmware I took a look into the missing hostname in syslog. I hoped to find an explanation but I've found a bug.
I've tracked it down to a bug in busybox version used in the firmware which means that small log format is...
What I would do to try and troubleshoot the issue or issues is firstly swap out what I'm going to call the internet facing router which is the AC87 and use the AC68 in its place and see how that goes for a day or two. Don't do anything with the AC87 in case you need to swap it back.
Next I...
Regarding dhcp-lease-max the history for 376.49 (21-Dec-2014) mentions
- CHANGED: Reverted to Asus's max-lease number calculation
for dnsmasq
The commit is
https://github.com/RMerl/asuswrt-merlin/commit/1c1eaaa7b1a6410ea0e8cec57e2876b2f139e7c5
For the script issue can you try this. Make the first line of the script the shebang with a -x for some extra debugging output.
#!/bin/sh -x
and run the script in ssh session. Remember to check that the script is executable. Does it work? If yes then just remove the -x.
Incidentally...
If your plex server is on windows it's likely windows will have detected the new router as a new network and assigned it to public network location rather than home. Try setting it to home in control panel -> Network and Internet -> Network and sharing centre
Try testing connection to plex...
After flashing new firmware try either of the these approaches
Use the full path to the script which assumes that the USB stick is named as per the QuickStart guide
/mnt/ASUS/nvram-restore-20141204-0480.sh
or to run the script in the current working directory use ./
cd /mnt/ASUS/...