I had a 3 piece eero set, with one at the router, and two peripherals. Router unfortunately has to be in a far corner. House is 2900 sq. ft. After setting up, I had TV going off line intermittently and lots of restarts. After being frustrated in correcting attempts, I called ATT and demanded a visit by a tech to analyze and correct the problem. He came. He saw. He conquered. First he turned the eero off and checked the signal strengths. First noted that I had the two peripherals too close together so that they would compete which results in the intermittent offs and ons. Second I had placed the peripherals in weak signal areas, and so what I was boosting was a weak signal.
He found two strong signal spots, as far from the router as possible, and as far from each other as possible. That took a little time, but turned out well. After 2 weeks or more, we have had no signal failures at the 6 TV's in the house. Signal is good even tho ATT wifi strength varies during high use times. The only remaining weak signal is to my wifi controlled smoker on the back porch, which is now improved with the better antenna.