I thought cashu relies on the mints lightning channels

That is really high! Weird. What mint is it?

Yeah this sounds like a lightning node issue. Did you not have an existing lightning wallet? Just FYI receiving small transactions to a new node with no channels is an expensive proposition. Your service provider probably has a minimum channel size and you are being charged for extra inbound liquidity. Best practice is to fund a new lightning wallet with a large on-chain transaction. Then spend your balance down naturally or swap it out with a submarine swap. That way you have a large channel balance ready ahead of time with inbound liquidity. This lets you avoid paying high time preference fees. Worst practice is to start with a small channel and have to increase the size of your channel each time you receive a payment. It sounds like you might be doing the latter. You might as well use Muun b/c you are transacting on chain for each LN payment. 😬