You probably can't fix that.
HTTP error 500 is an internal server error.
Server errors can usually only be fixed by the server administrator. To do so they would consult the server's error log. That log is not accessible to you.
If you are certain there is a problem with your router that is causing a badly written server process/thread to fall-over, you will have a hard time finding the cause.
One way is to use a network sniffer such as wireshark to capture traffic for a successful transaction on a "working" network and then capture the traffic for the unsuccessful transaction on your TL-WR740N network. compare the differences in detail and use your understanding of network protocols to identify potential causes of the symptoms.
But really, an ordinary router, using an out-of-the box configuration ought not to exhibit any problems. I'd maybe try a factory reset followed by absolutely minimal working configuration. Try a different router if you can borrow one (or swap with a friend)
It is very rare for any sort of configuration issue at your end to somehow trigger a fault in the server leading to a HTTP 500 response.