I am not sure how you can do this, but the Apple TV app is actually free, as is the Plex Server (in that you do not need to have Plex Pass to use it on the Apple TV).
Which QNAP do you have?
Maybe one possibility is to move your Plex server to another device (eg: PC, Mac etc).
It’s not Plex, it’s just how Apple handles App deployments. Regardless, there is probably not a fix for this besides compiling the release you need (if Plex even open sources this) and then side loading to your Apple TV. This is not easy and you would be better off just spinning up a VM or re-homing your Plex server on another computer that doesn’t limit you to an older version.
@voodle65: you could check if you can access the Plex Media Server using a DLNA client (e.g. VLC for tvOS). Alternatively you could consider adding a micro computer like a NUC to run your Plex Media Server while keeping the old QNAP as a storage unit (e.g. if you have a spare computer or don’t mind the invest).
On a more general note: If you can no longer update your Plex Media Server (e.g. because your platform does not support newer versions), make sure to disable automatic app updates!
This will not help the OP but should be an important consideration for other users with an outdated version of Plex Media Server they can no longer update.
Plex had announced this change some time ago:
in the release announcements for Plex for iOS / tvOS 7.4 (“this is the last version supporting servers older than 1.15.2”)
by sending mails where they could identify users of old servers (pre 1.15.2) as well as a dedicated announcement in the forum
The only Hardware solution now is to spend thousands of dollars on a new QNAP or more money on building a new additional system. Both are unacceptable.
The software solution is to run Infuse, which still works with Plex Media Server version 1.14.1.488.
Oh yeah, I bought a lifetime subscription with Plex so I could get EPG support. A few months later they killed it.
Sorry you cannot find a solution. For what it’s worth an i3 NUC would cost about $300 and would run PMS well.
InFuse is a good bit of software. A lot of Plex users (on this forum) prefer Infuse over Plex’s Apple TV app anyway.
The significant thing about PMS 1.15+ is that it is 64bit only, and QNAP devices that can’t upgrade beyond 4.2 are 32bit devices.
EPG support is actually still there, but (I think) now needs PMS 1.17.+ to work. Honestly, the new provider is beyond appalling (missing and wring episodes everywhere and always) and I have personally had to stop using it as I kept missing recordings.