I have trouble using the Core - Push based to get the price feed in Avalanche fuji network. I used the pyth contract 0x23f0e8FAeE7bbb405E7A7C3d60138FCfd43d7509. And gave price feed ID ETH/USD as given in the documentation.
Another method to fetch the price is getPriceUnsafe() If the price feed is available, the method will return the latest prices with timestamp of last update. NOTE: getPriceUnsafe() method does not check the freshness of the price.
To make sure you are getting sufficient price updates automatically, you can also run the Price pusher