Most likely the DSM upgrade wiped out your symlinks. Most of those types of hacks to get things working for (sort of) unsupported packages tend to not survive a reboot, let alone an OS upgrade.
Stop the Tvheadend service, recreate your links and ensure that the Tvheadend user can see the grabber in $PATH, then restart Tvheadend.
(Personally, the limiting nature of trying to get third-party apps to build, run and persist in Synology's limited Linux install was the reason I had to move to setting up Tvheadend on a small NUC-type of machine. Synology may publish an SDK, but it's definitely not user-friendly to figure out why your own custom packages won't work.)