To reach your device via 192.168.1.1 when connected to Hotspot you’ll need to append one firewall rule.
In WebUI → Network → Firewall → Traffic Rules find a rule called “Hotspot_input”, then click “Edit” button and by the field “Destination port” insert port 80 or 443 if you use HTTPS.
Appended rule should look like this:
After you save your configuration you’ll be able to reach your router’s LAN IP from Hotspot. Router's WebUI was made unreachable with default settings from Hotspot to compartmentalize network for security reasons.