daily restart of modem

I am in the 600MBps band and used to clock around 700Mbps at all times. 

Once the recent work from home in MI started, the speed slows down to 6Mbps or even slower. I have to restart the modem to get back to decent speed. I have to do this multiple times a day.

Noting much has changed in terms of modem/router, #connected devices. Same number of devices as during the weekend when we all connect our laptops & phones.

Why is this happening & what is the remedy?