io.netty.channel.ConnectTimoutException when connecting to Mineplex

io.netty.channel.ConnectTimeoutException means your connection has timed out with the remote server.

A server connection timeout means that a server is taking too long to reply to a data request made from another device. Timeouts are not a reply message: they show up when there isn't a reply and a server request is not fulfilled in a predetermined length of time. A server connection timeout error does little to tell you what went wrong or why the error happened: it just identifies that the error occurred. Timeout errors can happen for a number of reasons. The server, the requesting device, the network hardware and even an Internet connection can be at fault.

Source

What this means is either the server is taking too long to respond, it could have been blocked, server is unable to respond.

Since Mineplex is not just one server, but a network of server joined through Bungiecord, which is essentially a proxy server that acts as a gateway between you and the multiple servers on the network. This means your packets are relayed between you, the proxy server then to the mini-game server that you're on.
Based on assumptions, here are three possible reasons for your disconnect:

• The Mineplex network is busy and the sub-server doesn't receive the your proxied connection as the network is busy with other connections.
  In this case, try again later when there are less players online.
• Your internet connection between the main gateway may be terrible.
  There is nothing you can do, other than to get a better internet connection.
• The gateway server is online, but lobby server the gateway is trying to connect you to is offline, not responding to requests, or overloaded. *This is a very common problem and, being server-side, cannot be fixed. You will have to try again in 5-15 minutes and hope the gateway server has selected a new lobby to connect players to. It would be an odd coincidence that this happens to you on multiple/all the global servers but it is possible.

If you're certain your internet connection is fast enough for everything else, it may just be your connection between you and Mineplex.
Try tools such as a Ping Test or a Speed Test and ensuring your Firewall isn't set to block incoming traffic from *.mineplex.com.

See this image for more information on Minecraft Bungiecord server networks.