I have no idea of why QNAP makes the TS-228 = arm-x41, however there are numerous files, why not try each QPKG until one works?
arm-x09 http://download.twonky.com/8.5.1/TwonkyServerEU_8.5.1_arm-x09.qpkg
arm-x19 http://download.twonky.com/8.5.1/TwonkyServerEU_8.5.1_arm-x19.qpkg
arm-x31...