Odd that this would occur after the modem changed since you're not even connected directly to it. You're sure nothing else changed on the PC?
I guess if it were me I would check to make sure the link speed is still gigabit and that it's not somehow failing down to a 100Mb link. If the link is flaky I would start by swapping cables. If everything else tests out I would buy a discrete gigabit network card and run off of that.
I'd also try installing the drivers listed on the ASUS support page even if they are older. In the past I've experienced Windows auto updating to something newer but worse. In my case it was the Intel Management version that tanked my WiFi speeds and caused occasional crashes. I changed to another version and everything was good. Not saying that's your issue but it couldn't hurt to try ASUS's validated drivers. Unfortunately, if this is the issue Windows will keep updating so you may have to google how to disable driver updates.
I guess one last thing you could test just for sanity's sake is a bootable Linux live USB. You can run one of these like Ubuntu without installing anything and it would allow you to test your hardware with completely different software and drivers.
A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station…