First, please confirm these first:
1) Does your ONT/ONU have a Gigabit LAN Port?
2) Is your router, if any, connected to the ONT/ONU a Gigabit router or not?
3) What were the UL &DL speeds when it was working fine?
@ahamed Which plan are you using - 50 or 100Mbps plan? The speeds you mentioned above were on LAN or WiFi? If WiFi, then 2.4Ghz or 5GHz?
If your ONU does not have a Gigabit port, you won't get upto 100Mbps on LAN.