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