[Mesh] mesh routers can't opkg install kmod-usb-acm

[Jake]

i'm trying to use a mesh router to operate a duck coop door, and this means i
want the USB port to recognize an ACM serial port.  This is a normal thing for
openWRT and usually works thusly:

opkg update # downloads opkg package lists to /tmp
opkg install kmod-usb-acm # installs a kernel module for ACM serial USB devices

unfortunately, there are some problems with present mesh node firmware
(including a node that was flashed in December 2015 and another which was
flashed recently) and this process doesn't work.

for starters, the opkg.conf on mesh nodes is messed up - i'm not sure why but
there's a github issue here:
https://github.com/sudomesh/sudowrt-firmware/issues/108

to get "opkg update" to work, follow the instructions in the first comment.
That works for me, but when i do "opkg install kmod-usb-acm" i get the
following error:

# opkg install kmod-usb-acm
Installing kmod-usb-acm (3.18.23-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05.1/ar71xx/generic/packages/base/kmod-usb-acm_3.18.23-1_ar71xx.ipk.
Collected errors:
  * satisfy_dependencies_for: Cannot satisfy the following dependencies for
  * kmod-usb-acm:
  *  kernel (= 3.18.23-1-b2f200610f46d20ef52d269421369d0c) *
  * opkg_install_cmd: Cannot install package kmod-usb-acm.
# uname -a
     Linux jerkey-tl-wdr3500-2 3.18.23 #1 Fri Dec 25 00:14:49 EST 2015 mips GNU/Linux

i have no idea what this error means or why it's happening, can anyone offer
advice?

thank you,
-jake