Hi there. I got OpenWrt 18.06.1 r7258 succesfully installed on TL-WR842N v3.
Trouble #1 (or probably a bug): in the Interfaces>Switch i see " 1000baseT full-duplex" on CPU(eth0) aka WAN. This router has no 1Gb ports, afaik, and i see it even with blue and brown pairs cuts from the patchcord.
Trouble #2 (that maybe outcome from #1): transfer speed from WAN to LAN occasionally falls down to good-old-dialup-times (like 5-7Kbytes per second), only power cycle might help (or not). I leave it overnight on a good state, to the morning it comes back to bad, with no config changes from my side (and yes, change of config might be the reason to fall to that "bad" state too).
There is no way (that i know) to say "set 100mbit on that port" to OpenWRT, so maybe that'll help to troubleshoot.
Another fact - my ISP is [still] MAC-binding-enabled, so i forced to use "Override MAC-address" feature, otherwise there is no connection at all.
Apart from that, there is no non-default settings in my config by now. And right now it works OK, but i'm not sure for how long it will be.