Two days ago I sent the following transaction: 6f917d372316dead7ada76fdc7f2f854c7344df7d31c8a7bd6e454ede1110806
Your problem is this
unconfirmed input with 50 satoshi/byte, that transaction will have to confirm first.
I used the "economic fee" of 185 satoshi per byte (amounting for 5$ for an approx 50$ transaction).
I've seen lower fees than that get confirmed in the past hours, but your input's fee is still far too low.
Is there extreme traffic on the network at the moment?
There is, and has been for quite a while now.
I tried the first accelerator and I receive the message that "submissions are beyond limit". How could I find the right spot to submit? By trial and error?
I like to say "impeccable timing", it resets every whole hour, and you have at best a few seconds to enter your transaction. But you'll need to enter your unconfirmed input first.
As for the second, it asks for my mobile number, which I am not comfortable giving. Is there any other (free) way to push this forward?
ViaBTC is the best I've found. I've tried Antpool (just get a cheap prepaid simcard), but so far I can't say with certainty it ever worked for me.