Anyone is having success on this Issue? 
I tried what everyone tried but still cant.

@ZarinaAbdullah try 192.168.0.1 or tplinkwifi.net

@Mr_Whatsittoya After about a week of waiting TP-Link support finally remotely connected to my computer. After they confirmed that there was no way to fix it by changing any settings they issued a return merchandise authorization. 

I have similar problems on wifi repeater RE300. 

1. Can't online check firmware - Unable to connect to the cloud server. Please try again later.

2. Can't login to TP-Link Cloud - Request timed out

In rare chance that I could do above 2 actions after factory reset. Then timeout comes after few network configuration were done ie. set fixed IP, disable DHCP etc... So far these are my observations. But no big deal because I still could download firmware file manually and apply upgrade as needed. 

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Found a strange workaround. I edit primary DNS entry to other value and Save then put back 8.8.8.8 and Save again. The problem goes away. However after repeater rebooted need to do the same trick again.  

@EmailJim I'll probably just live with it.  Everything else works fine and I don't have any connectivity or performance issues.  I also can't remember when I purchased my C5400X but I'm sure the warranty has expired.  The power supply did die about a year ago.  I purchased and aftermarket power supply and it works fine.