ZeroTier versions after 1.4.6 do not work in ASUS RT-AC68U

I have 2 separate Home Networks which are about 20 miles apart.

HOME_NETWORK_#1

ASUS RT-AC86U Router
HW: arch64
AsusWRT with Linux Kernel: 4.1.27
ZeroTier package installed on the router via Entware repository.

RESULTS:
ZeroTier 1.8.3 version works without any problems.
ZeroTier 1.6.5 version works without any problems.
I can access the router as well as some IoT devices on the LAN.

HOME_NETWORK_#2

ASUS RT-AC68U Router
HW: armv7l
AsusWRT with Linux Kernel: 2.6.36
ZeroTier package installed on the router via Entware repository.

RESULTS:
ZeroTier 1.8.3 version does NOT work.
ZeroTier 1.6.5 version does NOT work.
ZeroTier 1.6.3 version does NOT work.
ZeroTier 1.4.6 version works OK.

For every version that does NOT work, I get the following message after the command:
zerotier-one -d

WARNING: ioctl() failed setting up Linux tap device (set MAC)

After this warning message, it looks like ZeroTier is “working” since the router can join my ZeroTier network, and the router node ID shows up as an active, authorized member in my network shown in the ZeroTier Central dashboard (https://my.zerotier.com/), so everything appears fine; however, the router does not respond at all when I try to connect to it via its assigned ZeroTier IP address. The configuration setup on this router is pretty much the same as the 1st router (in HOME_NETWORK_#1) which works very well. For some reason, the RT-AC68U Router works only when the 1.4.6 version is installed, and nothing else in the setup & configuration of the network is changed.

I really want to use the most recent ZeroTier version available to take advantage of all bug fixes as well as any security improvements.

What can I do to figure out what the problem is when using more recent versions after the 1.4.6 release?

Thank you.

I’m on the same boat. Two RT-AC68U. I try every upgrade and have to revert to 1.4.6.