Hi @RRich, welcome! You've come at just the right time. We have recently released all the relevant information for the so called ‘vrije modemkeuze’.  Which means you can now install any modemrouter you like (provided it support your type of connection). How is your Dutch reading? 

Start with this forum topic. It will give you a run down of how it works. Then go to this page on our website. Since you have fiber, and it’s just internet go to Stap 3A and choose Ik heb (bonded of vectored) VDSL2, VPlus of FTTH Glasvezel internet.  The top paragraph under Internet gives you all the information you need to setup your Asus. And, in answer to you specific question: there is no set username and password. Just fill in something (for example: internet / internet).

Now, the regular customer service of KPN does not support this, but you can get help from other users (and us mods as far as we know things) here.  So, if you have any more questions, post them here :)

Thank you very much for your reply and all clues.

As I mentioned, my Dutch is close to zero and in google translation I may lose some details.
Also, I am not a network expert and some this language not always clear to me.
The result is that I am still not connected. These are my settings of ASUS in WAN section:

From KPN instruction I am not sure what should be selected for IPv6:

The message says that it may be a problem.

I would be grateful if you could give me some further directions\instructions.

Sorry to hear you haven't gotten it to work, Rich. I'm gonna call in @wjb here as I can't quite see what is wrong here and he's the true expert on this. 

The pppoe connection needs to be made on vlan 6.

Is there an option on the Asus where you can define the vlan to use?

Thank you again for your quick reply.

I have checked the user manual and do not see VLAN settings: https://dlcdnets.asus.com/pub/ASUS/wireless/RT-AC88U/E10302_RT_AC88U_Manual.pdf. However, in ASUS support there is the following topic: https://www.asus.com/US/support/FAQ/1034366 but to be honest I have no clue if this applies to what you mentioned above. ASUS has an interesting online emulator of the router  http://demoui.asus.com/ that you can see WAN\VAN settings.

If you could be so kind and give me further support. Maybe any of forum members had similar problem before?

This router is not gonna work with your KPN connection. There is an article on tweakers. But in dutch thats mentions what asus router can be used with KNP.

https://gathering.tweakers.net/forum/list_messages/1772709

The asus router has to support the Merlin firmware and as far as I can see the rt-ac88u does not support this firmware.

So I don't think this is gonna work.

Do you have television from KPN or only Internet?

 Internet only. No TV, no telephone. 

Fiberglass connection.

Then it is possible to place a switch that supports vlans between the NTU and the Asus that transfers the communication to vlan 6 (tagged).

You could use the Netgear GS105Ev2 for this.

If I recall Correctly you should look at LAN > IPTV
here you can set it to Manual and then you are able to set your VLAN for your internet

This is also where you can set VLAN's on some other ports for IPTV and VOIP but you can also set the internet VLAN on those if you want to use those ports 
 

Edit:
The Simplest solution would be to set your ASUS RT-AC88U in AP mode and just connect it to the Experia-box (and turn it's Wifi off) and use it as an Access Point for your wireless connection

I really appreciate all your help but it looks like connecting my ASUS to KPN is over my skils

In fact, I am happy with Experia Box v.10A (signal and coverage are good enough, easy interface) and the only function I miss is a net-drive support. I connected external hard drive to USB but it is not recognized\detected by Experia. Is it normal?

If that might be possible, then I will forget about my ASUS and will be more than happy with Experia Box.     

The USB port can not be used to connect an USB hard drive.