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ISP :

I've tried looking this up and there really isn't much out there when it comes to troubleshooting the C5400 model.

Since I purchased this router 2 months ago, I've come to realize that Internet Explorer is better suited than Chrome when it comes to accessing the router management page through its IP address.

Recently, I noticed there was a firmware update while using the Tether App (which is very limited btw). After installing it, I lost the ability to access the management page from a web browser. I can still access the router through the Tether App but like I said, it's very limited and I can't setup DDNS or address reservation. Whenever I try to access the router's IP, it either does not load, loads the blue background and nothing more, or on the rare occasion, lets me login, but then freezes at main page after that. Everything is super slow and unresponsive. If I do a hard reset, everything is great, I setup every settings the way it was (I've tried doing backups and restore but also doing a fresh, clean setup) and eventually we get back to the same issue.

I have not added or removed new devices that connect to the router. I have about 18 devices connected at all time and I never had this issue until the recent upgrade.
This is getting super frustrating. The router operates fine (all my devices can connect wired or wireless) and I can access it through the Tether app, but web management does not work on Chrome, Firefox, Opera, Internet Explorer on a Windows 10 Desktop or even my laptop. It also gives me the same issue if I try to access it through a web browser on my Android tablet or phone.

I've tried reflashing the firmware but I honestly have no idea if it worked, the progress bar does not change throughout the upgrade and then the router just reboots.

Firmware Version : 1.0.9 Build 20170406 rel.53986(5553) Hardware Version: ArcherC5400 v2.0

Please tell me if it's a current issue with the firmware or if it's my router being faulty.

Update: After some further factory resetting and messing around, I think I found the cause of the issue. One of the 18 devices connected to the router is a Wireless Access Point. Mainly, my old TP-Link Archer C7 is setup as a wireless access point to be used as a range extender. Due to the shape of my house, I am unable to get a good 5Ghz signal in my bedroom from the Archer c5400 (or even the Nighthawk X6 when I had that). So I took my Archer C7, set its IP to 192.168.0.2, disabled the DHCP server and connected the two routers through their LAN ports (not the WAN for the Archer C7). This essentially made the Archer C7 work as a WiFi extender and lets the Archer C5400 handle distributing the IP addresses. During one of my factory resets, I decided to omit setting up the Archer C7 and only use the C5400. After setting up all my address reservations, DDNS and other settings, I rebooted the router. Normally, this is where I start getting problems. But somehow, I was able to access the Web page again. Then I decided to plug back in the Archer C7 and noticed the Web management page on the C5400 was slow or impossible to reach again. I unplugged the Archer C7 and the C5400 page loaded fast again, no issues.

So basically, the cause of my issues is my Archer C7 setup as a WAP. Now I still think this is a firmware problem because I've had this setup since I had the Netgear Nighthawk X6 and I've never had this problem until the recent firmware upgrades.