I'm facing a similar issue as the OP, but I'm using Hathway (now owned by Jio).
Around June my old Wifi
router conked off and I decided to get the
TP-Link Archer C6. Everything was working fine and dandy in the morning. I was getting full 50 Mbps on both bands (2.4 GHz and 5 GHz). However some time after 6 PM, the speed started to slow down and settled to 8-10 Mbps on both bands, including the LAN connection to my desktop!
Disconnecting the router and directly plugging the modem got the speed back to 50 Mbps again! But connecting through the router dropped it to 10 Mbps! Then, around midnight, the speed went back to 50 Mbps again, on both Wifi and LAN while being connected to the router.
Initially, I thought it was a router issue and decided to get a replacement, however even the replacement had the same problem, which led me to claim a refund. A couple of days back I got the
D-Link DIR 825, but I'm experiencing the same issue on this router too.
I'm pretty sure now that this is definitely not a router issue, nor is it an issue with my desktop or mobile. That just leaves Hathway as the culprit! Any ideas on why or how they might be doing this?