In a nutshell, the title say it all...
The modem, an asus DSL-AC68u works perfectly fine with any firmware up to 3.0.0.4_376_2187, which is the last one of the "3.0.0.4_376" serie. Since asus released its "3.0.0.4_378_xxxx" firmwares the modem started to show a quite weird behaviour.
I have a 90Mbps down / 10mbps UP vdsl connection (real speeds), and what happen with any "3.0.0.4_378_xxxx" firmware is that the throughput speed of the WAN drop to 15-45Kbps on clients connected to the modem by wifi (no matter the band). If you are wondering why I said "WAN speed" and not just "wifi speed", that's because the LAN wifi connection between clients and modem is stable, strong and fast too, with file transfers averaging 200Mbps within the LAN.
This happen only if the client is running a Windows OS, because two android phones works just fine, and also the same clients with with windows works fine if booted with linux. So far I've tested a windows 7 laptop, a window 8 laptop and an Intel compute stick running windows 10. Different OS, completely different hardware and drivers, same problem.
As for the DSL-AC68u, I've already replaced it once, with the exact same results: Everything works fine out of the box (with 3.0.0.4_376_xxxx firmware) but as soon as I update the firmware to 3.0.0.4_378_xxxx the problem is back. It's driving me crazy and ASUS support proved once again to be useless!
Does anyone have a clue about what I could check or try?? As much as I think about it, I cannot see what relationship there might be between the OS running on a client and how it could affect the WAN wifi speed, leaving the LAN speed unaffected.