[WIZ750SR] If I disconnect the LAN cable and reconnect it, I can not connect it.

There are several ways to resolve this issue, depending on your situation.
I will list the most frequently asked questions.

WIZ750SR series has keep-alive packet function. This function is to check if the product is physically connected.

For example, if the product is connected to a PC via a router uses a TCP connection, if the PC disconnects the TCP connection, the product may not recognize that the TCP connection is disconnected. This does not tell the product that the intermediate router has disconnect the TCP connection. In this case, the product recognizes that the TCP connection is still established. If this happens, the user has to reset the product and use it again.

The solution to this problem is the Keep-alive packet feature.
The user can configure the keep-alive packet function in the configuration tool.
You can also set the time to send keep-alive packets. (See photo below)

The same applies to the WIZ107 / 108SR product family.
In case of WIZ107 / 108SR family, Keep-alive packet function is not set to Default. If you want to use it, check the check box in Configuration tool.