How much two outdoor dbi should be to have a good connection?

I have two outdoor devices one act as an access point and the other one as a repeater my question is how much the signal strength should be( at least ) to have a good connection?

I can measure the strength between them both in (dbi) in the control panel of both devices

I got maximun strength between them 56 dbi and It works fine but I lost connection on the opposite side of the device

so if the connection between them both was like 35 or 30 dbi is it a good signal or what???