Question regarding ISP in Tokyo

I am using NTT Flets for around 1 year with no issues whatsoever, even during peak times I get around 1Gbps up and down, and around 5ms latency. Just today, suddenly my download dropped to 0.5Mbps, with no signs of recovery. Latency is still around 5ms and upload is still 1Gbps.

Technical details:

I tested on wireless and LAN, same result. I pinged my gateway and have steady <1ms and external pings are steady <5ms.

I tried two separate routers, same issue.

Of course I rebooted modem and router multiple times.

In my mind, it seems like an issue downstream. So today I called NTT and was told it must be an issue with my router, which is my own as they do not provide it, because they are seeing no issues in my area. They offered to replace the modem but doubted it would fix it, or send an engineer, both of which would cost at least 16000 yen.

1. I don't understand why I would have to pay for them to fix their infrastructure, is that normal?

2. How do I convince them that this is likely not my issue? I do understand that on the phone I'm speaking to L1 support, and it's not their fault, but it seems like they don't understand what I'm saying.

Did anyone have a similar issue and managed to fix it?