Cannot access certain HTTPS services (since 379..)

[zilexa]

Asus RT-AC68U
I upgraded to the latest version 380.61. Before that I was still on 379... but I have had this issue since I received a new modemrouter from my ISP:

certain services simply load forever when using them. It is always a secure site or for example my banking app on Android: I cannot login. A good example would be www.gmx.com, the most popular German email service. When logging in, it is forever loading.

The issue has to be the Asus RT-AC68U because when I turn on the WiFi on the ISP modemrouter and connect to it, I can access all these sites and it goes really fast.

What I already did:
1. I added the Asus local IP given by the Modemrouter as DMZ on the modemrouter.
2. I reset the Asus device, I did that again after upgrading to 380.61.
3. I disabled the firewall on the Asus.

Nothing changed, when I connect to the internet via Asus, some secure sites/services just don't work. Also in general I feel it takes a bit too long for a site to start loading. I know this sounds like a DNS issue, so what I also did:

1. entered Googles DNS servers manually on my Asus.
2. Used other DNS services (using Namebench to find the fastest one).
3. Switched back to auto (it should get DNS from the ISP modemrouter.

Again, none of this mattered. In all three cases I had the same issue.

I have no clue how to investigate this issue further. Please advise.

 

[zilexa]

None of that helped.
I can solve the issue by putting the modemrouter of my provider (Telfort, KPN with their Experiabox V8 device) in bridge mode. However, that same device provides routed IPTV to my settopbox. In bridgemode, I loose TV.

So I had to put the modemrouter back in its default mode. I did add the IP address assigned to the AC68u in the DMZ of the modemrouter.

Still, I have trouble accessing https sites, it takes a minute to load or just times out.

I tried disabling the firewall of the Asus, no difference. Is there anything I can do? I did not experience these issues pre-379 but I believe changes on my providers side also influence this issue.