I don't know any node-on-the-phone options that are using Core Lightning, so you will need a new full node, since your Umbrel is done-for.

My recommendation in that department would be Start9,

Then you need to get Bitcoin Core installed and start downloading the full blockchain. If your Umbrel was running on a Raspberry Pi, you will be pleasantly surprised at how fast the Start9 dowloads and validates the full blockchain.

Now you can install Core Lightning. You will need to transfer some sats to your node on-chain for setting up any channels you want, too.

As for who to open channels with: If you are only using this node to receive mining rewards from OCEAN, it makes the best sense to open a channel directly with them in the size you want to build up in your lightning balance before sending it off to cold storage.

Once the channel is set up, all the liquidity is on your side of the channel. That does you no good since you want to receive from OCEAN, so use boltz.exchange to either send the full amount to either your cold storage or back to your node's on-chain balance to set up more channels that you may want.

Now you are ready to create your offer invoice to give to OCEAN using either the Core Lightning UI or Ride the Lightning.