Here are things to consider. 

1. Please check the maintenance and service outage information of NTT. 
NTT East maintenance and service outage information (Japanese)
NTT West maintenance and service outage information (Japanese)

2. If you have had the ADSL modem switched on for a long period of time, it may be overheating, causing the instability. Switch off the ADSL modem, wait for about 10 minutes, and switching it back on for reconnection. 

3. Create a new internet connection setting. 

4. Check if you can connect to FLET'S Square (*). 

(*) NTT's FLET'S Square is a server within NTT's FLET'S network. If you can connect to it but cannot connect to Asahi Net, then please contact Asahi Net. If you cannot even connect to FLET'S Square, please contact NTT. You will not be able to connect to FLET'S Square with your Asahi Net User Name and Password, but you will need your NTT's FLET'S Square User Name and Password. Please refer to NTT's documentation. 

If you are using Windows 10 and you are suddenly getting disconnected, please refer to this page.