Just to clarify, 'auto' channel selection is effectively a requirement for connecting to upstream networks (I mean, if you know the channel, you can specify it, but that only works if that is always correct -- if the upstream side changes, your network goes down).
And the other element here is that a wifi radio used in sta + AP modes cannot come up in AP mode until it has connected to the upstream (sta mode) because this dictates the channel that will be used. As a result, if the channel is manually specified and is wrong (or if the upstream AP is not available), the downstream AP cannot start. That is why the travelmate package was developed.
Now, if everything is working properly on the upstream connection and your device is still unable to bring up the AP mode, this could very well be a bug/limitation of OpenWrt. A bug is certainly possible, but I don't know if the same bug I linked is still relevant.
In some cases, the functionality for this is supported in hardware and in the closed source vendor firmware, but if those libraries are closed source, they can't be included in OpenWrt -- in this case, it's not a bug, but a driver limitation due to not having open source code.
I also want to point out a curious statement from your conversation with TL-Link:
I've added emphasis to the word "original" because I wonder if it is possible that later versions dropped that capability due to a hardware change or other reason... I don't know, just raising this as a question.