The usb port (mini-usb type B) on the front of EX2300 switches are not enabled by default

To use it you have to

set system port auxiliary port-type mini-usb

then

commit
exit
request system reboot

After that you can connect to it using a USB cable

According to documentation from the Juniper web site:

Do not use the delete system ports auxiliary port-type command to delete the port-type configuration. Always use the set system ports auxiliary port-type type command to change the active management console port type.

To configure the RJ-45 console port as the active port, use the same procedure with the set system ports auxiliary port-type rj45 command.

However, I found that just enabling the USB port left the RJ45 console port enabled. YMMV!!

Tested on: JUNOS 18.2R3-S4.1 Kernel 32-bit JNPR-11.0-20200124.95c8806_buil  on an EX2300-48P

Reference: https://www.juniper.net/documentation/us/en/software/junos/user-access/topics/task/console-port-type-ex-series-cli.html