Solution provided by @flyride
I had to download the correct .pat file https://global.download.synology.com/download/DSM/release/6.2.3/25426/DSM_DS918%2B_25426.pat to make this work. Silly, but either I didn't supply any .pat file or supply the wrong one. Newbie mistake.
It worked like a charm and everything installed as intended.
Thank you for your help.