Hi, I have bough a 2-pack Deco E4. There are blind spots (low coverage signal of 3bars) in rooms in a condo, further away from the current 2 packs. 

Are the higher range model like M5 compatible with lower range E4 ?

Now I tested E4 is not as claimed to cover 2800 sq ft. My condo is 1000sq ft.

Should I proceed to add the 3rd unit of higher range, if possible or buy 3-packs E4 ? 

I am currently on Unifi 30mbps, due to copper wiring.

Will be upgrading to Time dotCom 100mbps after contract expire with Unifi.