I left the mempool dial where it was which was to the far left as possible, I didn't fiddle with it at all, are you meant to always adjust that dial?
You dont need to adjust the dial, the reason is most probably the amount you are trying to send is like the total amount and either you have left very little for the transaction fee which is below the required minimum fee of the server youre connected to or nothing at all. Just reduce the amount you want to send and leave just enough to cover for the fee rate it should go through. Check the links for fee estimation to know when they will drop.
It means you're paying fees to low, I assume the unpaid status because you send below the purging rates, it's either you wait until the fees drop and your wallet/other nodes to rebroadcast your transaction or you can create a new transaction with higher fees. If you have more coins, you can bump your transactions as long as the RBF enabled.
If the OP waits till the fee has dropped, he will have to broadcast the transaction manually himself because the wallet just saved it as an invoice. RBF wouldnt also work here because the transaction hasnt even be broadcasted yet.
The OP should have found a solution and the transaction may have been completed.
I also often experience this kind of problem because I missed the required gas fee and finally below the recommended gas fee.
But usually it will appear automatically on the gas fee that has been suggested or can be raised higher,
and the RBF system in Electrum is active by default without any more settings.
What you meant is transaction fee and not gas fee; which is associated with Altcoins, some newbies might mixed it up. As for the RBF it is only possible if the transaction has been broadcasted with low fees and its pending or uninformed. But this one still looks like a saved invoice.
Wait OP are you using the wallet with seed or private key. Hope it is not a watch only wallet because thats how it will save the transaction till you send it to where it will be signed (non-watch only wallet) and then broadcasted