Can't play on the same router (Xbox)

I recently moved in with my girlfriend, and we can't seem to connect to games together while our Xboxes are connected to the same router.

-This problem ONLY occurs in Apex Legends. We've tested other games like COD and Deep Rock Galactic, even Titanfall 2, and joining lobbies and matchmaking works without any issues.

-The router should be decent, and the connection as well. It's a D-link DIR-809 router with a 150mb/s download speed and 100mb/s upload speed.

-We can't get IpV6 connection to work because our connection is fiber. This really shouldn't be the solution though as other games work fine with the current NAT types etc
-We've tried portforwarding and "reserving" places for the static IP addresses of the Xboxes from the router settings, and toggling UPnP on and off, from both router and the PC.
-We've called our broadband operator for advice, which they couldn't give. They said it was peculiar that the issue only occurs in one game instead of all of them. They suggested to contact customer service etc of the game itself, as if that was a very realistic option.

Does anyone have any sort of a reliable fix for this? And please, if you do have answers, try to write some sort of a simple step-by-step guide with some simple English. We're not native English speakers and I'm not very knowledgeable with the jargon when it comes to networking... 

Such an annoying issue. I've tried to look for information about this online, and it all seems very scattered and inconsistent, some weird thing working for a person or two and not doing anything for the rest. I'd appreciate any advice. We just want to play together ☹️