Ok back to the explanation board, there has been some interesting development. I have to go lenghty as full transparency for fairness and accuracy.
As a reminder, I recently switched from ADSL (a poor old line averaging at 80mbs download) to Fiber. In the process I got a new router (zyxel ax7501-b0).
I'm supposed to have a 10G pon connection, shared with max 50 other customers according to Sunrise. My quick dumb and incorrect network maths made me realize 10Gbs/50=200mbs approximately. That assumption proved to be very coincidental, unfortunate and incorrect. Explanation further.
When it was setup, I had in the first few minutes tested the speed of the connection using my wifi6 mobile device, and saw something fast (like 600mbs or even 800mbs).
After that, I had to integrate that device to my network, that's when I discovered we could not change the DNS on that router.
Following numerous talks, checks and discussion with Sunrise support, they confirmed one can not change the DNS.
As explained aboved, I was able to bypass their router/modem restriction and change DNS but then the connection was apparently no longer working.
I reconfigured entirely my LAN, having to set static ips, etc... and changing from one subnet to two subnet, so I could use my own router, Netgear RAX200, to use my own DNS server.
Then I just received a new modem/router from Sunrise (Sagemcom fiber something), their support claiming I will be able to change DNS with that.
I have not had the time or will to try that device then, as it would have involved reconfiguring all again.
Timely, we are now few weeks, 1-2 months after the switch to Fiber, and I did speed tests again:
200mbs in average. I did numerous readings at various time, including night, etc... I also used different device and even cabled myself directly to the Zyxel using cat6a cable...
So I was unhappy with the 10G speed and recontacted the support after finding out that 10G PON is about (in short, shared connection).
After a few exchanges they told me that actually I'm having "Internet Confort M or something similar" and that by contract, the connection is limited to 200mbs.
That could not be... so I checked my sunrise cockpit/profile online and... that was correct. They proceeded with explaining me that while the connection line was able to received faster, my speed is limited due to that. So I informed them that is not correct, that I remember very well paying for a 1GB speed because I was commenting how I always paid for 1GB, yet the bad quality adsl line was capping me at 80mbs.
At that time I could not find email evidence of the above, so I asked them to give me some time to call them back.
I verified my contracts and indeed, my contract is supposed to be "Sunrise Internet Max Giga" (supposed to be max 1gbs download max, same for upload). That "Sunrise Internet Max Giga" naming no longer exists in terms of subscription from that ISP Sunrise, in Jan 2022.
So at some point, without informing me, my subscription seems to have switched from "Sunrise Internet Max Giga" to "internet confort M or something similar". Prices and everything remained the same.
With that information at hand, I call back, explain and a solution was proposed: Sunrise offered to change me to the "We home XL" (which is the actual 10G pon thing), which allegedely turns out to be even a few francs cheaper than my current subscription... Apparently without any incidence of my subscription (no extension of contract duration, time, etc...) beside being...a few francs cheaper.
So I accepted, I am yet to see and confirm it all went ok, which I will only see with the next bills.
The only mystery remaining was how come my initial speedtests were around 600/800mbs, if I was capped to 200mbs. My theory is that quickly after the initial connection the modem gets firmware/profile from Sunrise, and even updated its firmware and rebooted. I guess my speedtests were done before my speed was capped or something similar...
Conclusion: the 10G pon thing is not limiting me to 200mbs. The fact that there were allegedly shared between 50 customers was also unfortunate because that did not explain constant 200mbs speed. The explanation of that capping because of my subscription which was changed without any notification or price change in the past 2 years. That subscription limited me to 200mbs download max.
The problem was I am paying for "Sunrise Internet Max Giga" supposed to be up to 1GBS. Sunrise support made me an offer and I will allegedly get the 10G thing for a few francs cheaper than my current contract. I accepted.
At this point I do not know which one is better, the Zyxel or Sagemcom. I do not know why the Zyxel is not allowing us to change DNS, especially if the same connection with the Sagemcom allegedly can... I will probably not go through reconfigure everything again and probably return the Sagemcom (unless I do some research and if I find that model might be better than the Zyxel).
Zyxel support told me one can change the DNS; but with Sunrise firmware we can't. Sunrise has not been willing/able to explain me why there is such restriction.