Sounds like maybe you don’t have any incoming liquidity. If you open a channel, all the liquidity will be outgoing until there is a transaction that moves some satoshis from your side of the channel to the other side of the channel.
Can you post a screenshot of your open channels in your Umbrel dashboard?
To start, you’ll want to connect to a few big nodes. You can search at 1ml.com - Wallet of Satoshi is a big one https://1ml.com/node/035e4ff418fc8b5554c5d9eea66396c227bd429a3251c8cbc711002ba215bfc226
Then if you send some sats from your Umbrel to your Muun wallet, you should see the liquidity in your channel change, so there is some balance on both sides of the channel. Once there is balance on both sides of your channel(s), then sats can flow into your Umbrel.
Let me know if that helps get you started.