Question Optimizing Home Mesh Network

[drpresco]

I have AT&T Fiber internet 500Mbps speed with the Arris BGW210-700 with a google mesh network with Google NLS-1304-25 with 4 units. I am trying to cover my property that includes a 2800 sqft house and a detached garage with apartment that is 120 ft behind my house. 3 units are in the house and 1 in the back garage. There are dead spots in the house and unstable and slow internet in the garage. I am looking for recommendations on how to fix connection issues or completely change my system. There is a cat 5 wire from the house to the garage if that is useful. All recommendations are appreciated.