IT'S POSSIBLE UPGRADE TO PHP 7 on NAS326?

Shaker

What I'd propose is not exactly an answer to the question, but I would rather become independent from the NAS php version and set up a Raspberry PI with whatever php version you like and use the NAS only as a storage and database. Webserver and PHP can definitely be "outsourced" where they can be maintained more easily.

zelgit

https://community.zyxel.com/en/discussion/comment/63306#Comment_63306

I agree and is something I'm thinking of pursuing when it comes to Let's Encrypt and Nextcloud.