@Gabriel1 

Please refer to this guide to confirm the IP address and gateway on the computer when it is connected to the WR740N with an Ethernet cable, disable the wireless connection on it if there is one:

https://www.tp-link.com/support/faq/841/

Then try to login again by inputting the gateway on that computer in the address bar of the browser, check again if you can login. If still not, can I have the screenshot of the error page?

Use Method 1 to reset the router one more time, ensure the LEDs flash off then on, that means the reset is successful.

@Gabriel1 

Hello, thank you for the response at first. So i followed the first guide and those are the results:

I did restarted the router few more times, but the LED indicators don't seem to respond so much, I mean all of them are always fast blinking except the one for the power, but it's important to say that it has always been like that since I own the device and I did restarted the router before and it never been a problem. 

I'm judging that the device is reset when it changes it's name and password to the default ones in the Wi-Fi bar on my phone.

Here is the screenshot of the error page:

I tried the suggested solutions in the error page, but it doesn't seem to solve the problem.

@Gabriel1 

Try different web browsers and end-devices (computers or phones). If you only have one end-device, like a computer, attempt to restart the computer or clear DNS cache on the computer (click here).

Also, disable firewall or anti-virus software on the computer, then try again.