If all three are really the same models (i dont know the difference between v2.0 and v2.1), i have cloned many of them and never ran into a problem; but my models are really all identical, all bought in 2015 and they all state "Model: Archer C7 Ver 2.0" on the label.
However you copy /dev/mtdblock3:
For LEDE 17.01 or Openwrt 18.06 i always copied the complete firmware (/dev/mtd5, check your /proc/mtd to be sure; on newer firmware or the now available V5 which isnt compatible to the image anyways, the layout is different).
Maybe something on your router was different initially and is not overwritten that way (like kernel doesnt match module version ect.), thats the only suggestion i have.
After cloning, remove
/lib/firmware/ath10k/cal-pci-0000:01:00.0.bin
/lib/firmware/ath10k/QCA988X/hw2.0/board.bin
(will be rewritten after the next reboot)
...otherwise the Wifi MAC address of the cloned router is not its own but the one of the clone source, this is only a real problem if you run multiple routers at the same place.
Additionally i always save any routers "art" partition, so i have the original just in case something bad ever happens.