Nextcloud download slow on WAN


I am using the Nextcloud snap which was installed automatically during Ubuntu 18.04.4 installation.

In my private home network I got speeds at around 200 MBit/s to the server thru Firefox but:

  1. why not Gigabit like I get via FTP? (acceptable problem)
  2. When I connect via WAN I have serious download problems… that is a very big problem!

I will describe 2) a little more:
My internet-speed is 300 MBit/s down and 30 MBit/s up, so it will the the other way around when I am somewhere on this planet.
Uploading a file from outside is no problem, the bottleneck is the upload-speed from the network, where I am and not from my home-network but when I download a file from somewhere else I do not get 30 MBit/s!
I only get 3 MBit/s in average and there is something curious: There is the speedtest-cli which has a bug, that it shows very slow upload speed but this application shows exact the right speed that I get thru Nextcloud from WAN.

My router is synced with no-IP, so I have got a domain name and the http/https ports are forwareded to my server…

Can anyone help me to diagnose this problem?
If you need further information please tell me and I will try my best to provide them!

Ubuntu 18.04.4
Nextcloud Snap 17.0.3
i5 9400
Samsung Data-Center SSD

Edit 25.02.2020 10:46 CET:
I tried python speedtest-cli:
Testing download speed…
Download: 284.63 Mbit/s
Testing upload speed…
Upload: 29.58 Mbit/s
But why do I get only the speed from the bugy speedtest…??

Edit 25.02.2020 18:38 CET:
I found out that I get 500 MBit/s up and down when I am in the network and use the IP of the server in Firefox instead of its domain.
When I use the domain I only get 200 MBit/s as mentioned before.

Edit 25.02.2020 23:51 CET:
I checked the upload-speed in nmon and the server sends exactly the speed that arrives at my download-test-person so no data gets lost… must be a bottleneck

Edit 28.02.2020 17:35 CET:
My ISP told me, that the issue must be on server-side…
I found out that also nginx-test limits at 6 MBit/s Upload when I try with a large jpg in the demo-html-page.
At this point I clearly do not know how to go on, I would just try again from scratch and purge the whole linux OS…