SPK for Synology is available here:
https://synocommunity.com/package/radarr

Synology DS414 uses armv7l CPU MARVELL Armada XP MV78230

Manual update when auto-update fails:

sudo synopkgctl stop radarr

cd /var/packages/radarr/target/share/
sudo mv Radarr Radarr-old
sudo mv /tmp/radarr_update/Radarr .
sudo chown -R sc-radarr:radarr Radarr

sudo synopkgctl start radarr

Issue updating Radarr with error

[v3.1.0.4893] System.UnauthorizedAccessException: Access to the path '/proc/19002/maps' is denied.

can be solved by stopping Sonarr first, applying the Radarr update (manual stop and restart may be required) and then re-starting Sonarr.

sudo synopkgctl stop nzbdrone
#apply web update here
/tmp/radarr_update/Radarr.Update `ps ax | grep "Radarr/bin/Radarr" | grep -v grep | cut -d " " -f 1` /tmp/radarr_update /volume1/@appstore/radarr/share/Radarr/bin/Radarr

sudo synopkgctl stop radarr
sudo synopkgctl start radarr
sudo synopkgctl start nzbdrone

Radarr update files:

wget --content-disposition 'http://radarr.servarr.com/v1/update/master/updatefile?os=linux&runtime=netcore&arch=x64'
wget --content-disposition 'http://radarr.servarr.com/v1/update/master/updatefile?os=linux&runtime=netcore&arch=arm'
wget --content-disposition 'http://radarr.servarr.com/v1/update/develop/updatefile?os=linux&runtime=netcore&arch=arm'

To change the favicon if multiple instances are running, the following files can be changed:

/var/packages/radarr/target/share/Radarr/bin/UI/Content/Images/Icons/favicon.ico
/var/packages/radarr/target/share/Radarr/bin/UI/Content/Images/Icons/favicon-32x32.png