According to OP's last message, the transaction in question has since been confirmed without any sort of transaction acceleration service or utilizing the RBF feature. In hindsight, the accelerator service is hard to get in since it's capped per hour and if the mempool is free, no need to accelerate a tx.
Yeah I remember trying to use it back in 2017 and gave up. You needed to set an alarm and at the beginning of the hour try and get your transaction in, most of the time it timed out or it was already full. Eventually gave up and paid the $50 fee to send a small single input single output transaction.
This is why from now on I always have the RBF enabled , so if after a day or two it still doesn’t fill I can up the fee and resend it. In the past I got burn this way by having transactions stuck for days.