There's also possible to plug a SPF module and get rid of the ISP's glass fiber box completely, see https://github.com/xvzf/zyxel-gpon-sfp
As you can see in the article, it's possible to buy this module online , at Telekom's website - https://geschaeftskunden.telekom.de/internet-dsl/produkt/digitalisierungsbox-glasfasermodem-kaufen for a fairly good price and make some "adjustments". But I personally have not tried it.