Yes you can.

If you have TV from KPN then @Babylonia can tell you all about it.

If you don't have TV from KPN then you just have to make sure the Synology will use pppoe on vlan 6 to connect to the Internet. The login and password may be anything.

Hello  @HanaR 

Yes you can use your Synology router directly connected behind the fiber “ONT” box.
(= fiber to ethernet conversion).

There is a Dutch Synology community website, however everyting is written in Dutch.
https://www.synology-forum.nl/synology-router/rt6600ax-router-setup-en-zijn-mogelijkheden/

If you are using Microsoft Edge web browser, it could be possible that you have the possibility to translate the Dutch webpages immediately for better understanding.

Internet subscription only.  Just what @wjb already wrote:

(Synology router SRM version1.3.x examples).

1.  PPoE connection - < this page >  and  < THIS IMAGE >
   Login name and password can be anything.
    
2. VLAN 6 setting  -  < same page >  and  < THIS IMAGE >  (no IGMP proxy set)

In case you also have a  KPN IPTV subscription. (I doubt it is an interesting offering for you).
Another option should be set by using a special named “Profile”.
But it could be that this setting is not within your Synology router model ??
Depending from your previous country where you was living, within another part of the world.
And the Synology router is bought there, using another firmware / hardware base set offering??

But in case:  KPN Profile  -  < same page > and < THIS IMAGE >
Check marked for:  IGMP Proxy  and  IGMP  Snooping

Hope this helps.
If any other questions, feel free in asking.