Aha. But the wifi is served by the pi and is not related to any router. Only the LAN cable is to a house router. The wifi is from the pi via service:
[Unit]
Description=hotspot
After=network.target
StartLimitIntervalSec=0
[Service]
Type=simple
Restart=always
RestartSec=1
ExecStart=nmcli device wifi hotspot ssid <hotspotname> password <password>
<etc.....>
[Install]
WantedBy=multi-user.target