I have the DOD mini expression pedal
https://digitech.com/en/products/dod-mini-expression
works perfect, you need to set it to TS mode.
Expression pedals for the helix need to be the TS type ( Tip / Sleeve ) rather than the far more common TRS type ( Tip / Ring / Sleeve ) .
I didn't realise this at first and found TRS pedals could work fine one moment, then start doing odd stuff then next, so whatever pedal you get make sure it is a TS type with a resistance of around 10 to 20K.
That's why people go for the mission pedal, its set up correctly for the slightly odd way line 6 kit works.
If you cant find a TS pedal you like, you can try running a TRS pedal but using a standard mono lead to connect it rather than the more usual stereo lead expression pedals use.
That will convert it to TS, but its a bit of a gamble if the resistance will be to high to use reliably.