I have been struggling with my ISP to fix a speed issue on my Internet connection.

I have 100/10 connection that worked just fine until a few weeks ago. For no apparent reason the download speeds decreased drastically and after some local testing I decided to completely change my router and to redo the cabling, I purchased an Asus RT-AC66U and some Cat 6 Ethernet cables but it didn't changed anything.

I contacted my ISP and they sent some guys to do some testing, the bottom line is if connected directly to the modem the speeds are perfectly fine but if the traffic goes through the router speeds drop by at least half. They also brought their own router and the problem remained the same so it can't be my router specifically.

They changed my modem for a new one, no changes, they changed the internet plug for a new one and made some fancy measurements, they don't see anything wrong, we changed all cables, tested different cables always the same thing.

Even the ISP network admin came to my house to see for himself but couldn't find anything wrong, they left this morning after a 4th visit saying they would look into it but it wasn't going to be easy, doesn't sound good to me.

Anyone has any ideas?