Hi!
Please do as per below:
1. kindly check if your PC get the same subnet ip address 192.168.18.X with the ONT;
2. if your PC didn't have the 192.168.18.X subnet ip , please change your NIC address manually;
3. try to change one browser to log in to the ONT;
4. if you still cannot log in to the ONT, it means your ONT IP pool has changed from the uplink server. Connect your PC to the ONT, set the IP to the DHCP mode and then check the default gateway.
I hope it will be helpful for you. Thank you!