For other people in the same situation (or future me) I solved it by:
- Going into the Qnap with Winscp (do not forget to tick the option to show hidden files)
- go to /share/CACHEDEV1_DATA/.qpkg/TVHeadend/config (CACHEDEV1_DATA probably is changing between devices)
- open the accescontrol folder
- open the numbered files and change this setting into: "prefix": "127.0.0.0/24,192.168.0.0/16",
- save
- restart TVHeadend in the Qnap interface