Start VNC-Viewer via python script on raspberry pi os buster

Question

I am total python/linux beginner and struggeling starting a vncviewer on raspberry pi with buster via pythonscript...

I made a configscript so 1 image can fit in about 20 places. That part works great. Now we got problems with not reconnecting vnc viewers, after complete restart everything works fine... so i thought it would be a good practise to create a pythonscript to monitor the vncviewer and restart just the viewer if needed.

# Importieren der benötigten Module
import time
import subprocess
import socket


#aus  IPAdresse zu PCnamen finden
def get_ip_address(hostname):
    try:
        ip = socket.gethostbyname(hostname)
        #print(ip)
        return (ip)
    except socket.error as e:
        print (str(e))

#VNC Server aus der Datei lesen die vom Configskript mit entsprechenden Daten gefüllt wird
def read_vnc_server():
    with open('/home/pi/vncremote.vnc', 'r', encoding='ISO-8859-1') as file:
        lines = file.readlines()
        second_line = lines[1].strip()
        #print(second_line)
        hostname = second_line[5:]
        #print(second_line)
    return (hostname)


# netstat Verbindungen nach erwartetem Servernamen filtern, wenn gefunden dann verbunden und return true
def test_connection(vnc_server):
    ps = subprocess.run(['netstat', '-an'], check=True, capture_output=True)
    vnccon = subprocess.run(['grep', '-e', vnc_server], input=ps.stdout, capture_output=True)
    #print(vnccon.returncode)
    return vnccon.returncode == 0
   
    
while True:
    hostname = read_vnc_server()
    #print(hostname)
    ip = get_ip_address(hostname)
    #print(ip)
    ip = ip + ':5900'
    #print(ip)
    #print(test_connection(ip))
    if not test_connection(ip):
        subprocess.Popen(["/usr/bin/vncviewer", "-config", "/home/pi/vncremote.vnc"])

    # Warten für das definierte Zeitintervall
    time.sleep(10)

If I run that script using geany in raspberry pi os everything works fine, but starting it as a service will always end up with "unable to open display"""

so I think this line is the problem: subprocess.Popen(["/usr/bin/vncviewer", "-config", "/home/pi/vncremote.vnc"])

I tried some display:=0 arguments in that line without knowing why...

xhost +localhost

nothing worked

It should be a problem with starting out of shell/service, when starting in raspberry pi os everything works fine. But cant figure out the problem.

Can you help?