FYI: However, to install your own Fiber modem (SFP module) you need the necessary information from Proximus!
But whether they will give it to you is another question!

@Cemilaw 

Some additional information:

Here you can also find the answer from a Proximus employee: → Eigen Fibermodem | Proximus Forum :

“This is not possible for the time being, BIPT has announced that they will make a decision on this by the end of 2023.
There is indeed information available from AVM that this is already possible in various countries, but so far Belgium is not one of them.”

We have to wait to use our own modem till end 2024, then must every provider in Belgium give a solution to use other modem than the one from the provider, but I think when you use your OWN material the privider don’t give no support on it when you have a problem.

@Cemilaw regarding the comment from Master-Magic :

BIPT opens the market for modems BIPT has established the network connection point (NTP) for fixed networks in a new decision. The NTP determines from which point the user has control over his internet connection. BIPT views modems and routers as part of the end user's terminal equipment, meaning they can be freely chosen for broadband and internet telephony. As a result, users are no longer obliged to purchase the modem/router from their operator when taking out their internet subscription. Download document "BIPT opens the market for modems"

In order to ensure smooth development of modems by interested parties, all operators the characteristics of the NTPs and the technical specifications of the modems and routers publish six months after the entry into force of this decision. Within a period of twelve months After the entry into force of this decision, operators must make the necessary IT adjustments follow from this decision. End users must therefore do so twelve months after the entry into force of this decision, which amounts to November 1, 2024, can connect their own terminal equipment to it network.