That’s not a gateway, but a non-compliant device which leads to misoperation of the network in a way that negatively impacts other users, not just yourself. Please disconnect it immediately.
But I can’t find any example code for using the Oled to display the RSSI and some Infos like State of Connection.
Once you’ve stopped interfering with the shared network by running a non-compliant device that pretends to be (but is not!) a gateway, look for the previous thread here on using such a display.
You’ll then need to consider where the display is, and what information is knowable to it.
A node generally doesn’t have any RSSI to show, because downlink events are rare.
Nor does LoRaWAN have any real concept of a “connection” - you could show details such as OTAA session state, or ADR state relatively to ramping up or down data rate, but these are fairly distinct.
A typical actual gateway sees traffic at too high a rate for a display of individual packet RSSI to be very meaningful, what is useful there instead is things like the state of the backhaul connection to the network server, and statistics about total packets received or sent, their pacing, etc.