You bought an obsolete system, the UC5xx are past their EOS/EOL date, the last CME release they received was 8.1 or 8.2, while at the same time buying a current device which was never tested or supported on those releases.
7841 needs CME 10.0 as minimum to work
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucme/feature/phone_feature/phone_feature_support_guide.html#_Toc526926965
https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucme/requirements/guide/33matrix.html
You need to either buy old devices which are compatible with the old CME release you have on the UC5xx, or to buy a newer ISR which supports a CME release that has the 7841 included.
HTH
java
if this helps, please rate