Softbank hikari Internet Dropped too often

So I’ve been using the softbank internet for 6 years now, and recently I bought a TP link ax5400 and the internet was faaaast, like 100MBps download but when I downloaded something with that speed for like 5 minutes, the internet dropped, and I had the restart my router and leave it for like 10 minutes in order to get the internet connection back. I’ve been dealing with it for a long time now.
Does anyone know what’s wrong? It wasn’t the router’s fault,I tried other router, same thing happened.
Thanks in advance 、よろしくお願いします！