High latency caused by router
I have a D-Link ADSL modem (2740u) and TP-Link C7 wireless router. The modem is responsible for internet connection and the router provides an access point. The router is connected to the modem by an ethernet cable and nothing else is connected to the modem. Recently, I have very high latency (something around 3-5s on pinging 8.8.8.8, similar on other destinations). At first, I thought there's something wrong with the ISP but they checked and told me everything seems fine from their side. I checked almost any moving part in the network. Everything seems so fine. I have stable connection to internet, connection to router is fine (latency is less than a millisecond) and the same for the modem (latency is less than a millisecond). I connected to laptop to the modem with an ethernet cable directly but the ping was so high. Accidentally, I removed the ethernet cable of the router and it instantly decreased latency to 70ms (from 3-5 second)!
I'm sorry if it sounds so dumb but I think it's the root cause. These are what I tried so far but still ping is so high.
- upgrade my router firmware
- reset the router to factory settings
- change MTU (1500, 1400, 1300) of the connection between the modem and the router.
- change the modem to TP-Link VR400.
- change the cable between modem and router
It would be great to have your suggestions.