Well, in the end I had to do some soldering and follow the (rather brief) instructions to install via serial, which worked.
Don't know what's going on with the OEM firmware, but my version was newer than any downloads listed on the TP-Link site. Anyway, guess they've added new checks or something, so don't go expecting the straightforward installation documented on the wiki page.
The fact you were able to install the present OpenWrt images suggests TP-Link implemented a version check. Those checks are worked around in the TP-Link safeloader code, part of firmware-utils:
Specifically, I think .soft_ver might need to get set, to something higher than 1.1.7. OpenWrt uses 0.0.0 by default.
Normally, trying the OpenWrt installation from the OEM web UI would throw a more helpful error message on the serial console, so it would be helpful if you could revert back to OEM firmware to try and install OpenWrt once more. That error might then suggest what minimum version TP-Link expects on newer firmware.
I also got the EAP615 with the 1.1.7 FW. I am still on original FW and would be able to connect the device via serial. Can you point me directions how I can obtain the logs necessary to get more info about the firmware version that is expected?
Unfortunately, nothing happens when doing so. I only get a busybox instance that does not give any logs or other outputs when I try to install OpenWRT from the web UI.
Are there other options to obtain error logs?
Edit: Wouldn't it suffice to bump the soft_ver to something like 1.1.8 for the update to be accepted? I have no experience in building OpenWRT firmware with patched firmware-utils. If someone would give me directions, I would be happy to try it instead of soldering...
Edit 2: After some research, I found a hacky solution, that lets me install OpenWRT without soldering. This worked by using the same trick @mrm used on the TP-Link EAP225 [here]. You basically directly edit the OpenWRT Factory Binary with a hex editor. I changed the string
Can you compile OpenWrt yourself to include the firmware-utils patch? If so, would you mind testing first if compat_level is all that we need to bump? If not, there's a second patch below that does bump the compat_leveland sets the soft_ver to 2.0.0.
Either of those patches needs to go into the tools/firmware-utils/patches dir in your buildroot, sth like 200-eap615-test.patch e.g.
I will try the build tomorrow with the respective patches and report back here.
Edit: @Borromini I successfully built the OpenWRT firmware with the first patch and I am happy to confirm that the device accepts the firmware now. Thus, only compat_level needs to be adjusted in order for it to work.