terrible wifi

Hello - i have had comcast/xfinity for over 20 years...  as of the past year, the wifi is absolutly terrible.  we have moved the modem to has many places in the house that we can, with no avail.    earlier this year we dropped cable and went to streaming so just have internet - and upgraded to 1000mbgs!!!  when i do a speed test i never get over 350mbgs, and i'm paying for 1000.  any suggestions would be greatly appreciated.