I have no issues with Wireless syncing. The trick is
1 - enable port forwarding as you have done
2 - do NOT connect TO THE IP ADDRESS that calibre displays. This is the IP address within the virtual environment that calibre is running in. Rather you need to connect to the IP address of the Chromebook.
In my case, calibre shows me the address 100.115.92.205 with port 8080 (for calibre content server) and 100.115.92.205 or 172.17.0.1 with port 9090 (for wireless device connection).
However the Chromebooks IP address is 10.1.1.191. I have forwarded ports 8080 and 9090 to 8080 and 9090 via TCP. Remember you need to re-enable port forwarding after every time you restart the Linux environment.
From my Kobo I can connect to
http://10.1.1.191:8080/mobile and from an Android device using Calibre Sync I connect to either 10 1.1.191 port 9090 for a Wireless Device or
http://10. 1.191:8080 for a Calibre Content Server connection.
I don't currently have the ability to access from outside the LAN but all that would be needed is to forward those two ports from my router to the Chromebook's IP address.
I don't use KOReader but the same principles of the IP address will apply to it
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