Had or have? If you still have the TS-251C and try to install the app on it, please note that it does not have the minimum required amount of RAM. This might be the reason why installation fails for you but even if not, I don’t see the point for trying to run Roon Server on a NAS with only 1GB of RAM.
AFAIK does QNAP not provide any version of Roon at all (Roon is not bundled with the RoonServer app) and the build number you posted is related to Roon. The RoonServer app releases are distinguished by theire release date.
Why do you want to install the old legacy version of Roon instead of the current one? What do you think will be the benefit of this? The hardware requirements are the same for legacy as well as current Roon Server.
How did the installation for the packages fail for you? Please show us the error messages / log contents of the failed attempts.
Qnap does provide a roon download in their App Center, but it is always current production build, no legacy. If the manual installation of an old version fails, I would take it as a hint that either RAM or OS version are not sufficient to run roon on that machine. 1GB of RAM is simply not enough to even start roon. Starting roon on a Qnap with even 2GB of RAM might lead to complications.
I would not even try to run roon on such an old and CPU/RAM-wise lean machine. My recommendation: Get yourself a (used) Qnap TS-251D. That is an affordable and comparably simple multimedia NAS with a suitable CPU allowing up to 8GB of RAM and latest QTS. Roon runs surprisingly smooth on that one if you do not have an extensive library or demand crazy multiroom stunts. You might even be able to migrate your hard drives.