There currently is a firmware issue that incorrectly inserts the name for delay option into configuration file from WebUI. The issue will be resolved in the upcoming firmware versions.
For a temporary solution, to set the delay you want, you can use uci commands from routers CLI. You can find the connection methods here.
After logging in, input the following commands:
uci add_list gps.https.interval='<any interval>'
uci commit
This will then make the HTTPS server to send data on the set interval instead of the default 5 seconds.
If you want to change the interval at any point, you will need to use another command:
ucit set gps.https.interval='<any interval>'
ucit commit
Regards,
Paulius