I have searched around for a similar problem but I couldn't find any that replicated mine... So here goes nothing... 

I am with MyRepublic, in QLD. (Cairns area more specifically) I have had a room mate who we used his Router, Im pretty sure it was a Nighthawk XR1000. Port forwarding worked on that and I was able to play Elite Dangerous with no issues with friends. 

Room mate moved out, I bought a TP Link AX1500, did basic setup, tried to play some Elite, then suddenly theres a constant connection error, but thats okay it would still allow me to play. But after an hour I realized I cant see any of my friends, or join my friends. I can see their icon, but I cannot interact with them. 
So I assumed it was a port forwarding issue, which it turned out it was (as confirmed by messaging the devs), but whenever I setup portforwarding with/without DMZ, not a single port of mine can be accessed. Using canyouseeme.org literally none of my ports are opened, even if I set the ports to be opened. 

Windows Firewall is off also. 

So at this point, I messaged MyRepublic, which they got me to get a static IP. Didnt work. 
Messaged MyRepublic and now i have the "gamer" package. Still doesnt work. 
Messaged MyRepublic and they told me to get in contact with my router support. Which I did, they said it was my ISP (MyRepublic). Alright then. 
Messaged MyRepublic, told them, screwed around for 5 hours, nothing. 

Alright Ill try another router. Didnt work. 
Maybe two bad routers? Got another one. Didnt work. 
(note, all routers I got were different, I borrowed both, one being a NetComm N300 and the other was another brand, don't remember the model) 

And now as I'm typing this, I'm now buying a "MyRepublic certified" router to see if this will fix my issue. But I don't know. 

So Im at a loss. 

Im not exactly great at networking, I was a student for a while, but doing CISCO certs is somehow more easier to wrap my head around than doing it for a home network. 

Any ideas? Im more than happy to provide any details needed.