How to escalate an online ecommerce dispute

There's a brand online called Tradeinn that has lots of sub-brands (running gear, trekking gear, hiking gear etc). I've bought several times from them before. Anyway, long story short is I'm getting *nowhere* in seeking a refund and I'm stumped as to how I take it from here? The brand is registered in Spain so is within EU.

Basically I bought trail running poles in July, value approx €130, I specifically opted into a guaranteed delivery of 6th Aug. But when I flagged it on the 4th Aug that the tracking link stated something to the effort of "No consignment on record" (usually this means the order hasn't arrived at the depot) they said they were investigating.

A week later, since the poles hadn't arrive and the order hadn't updated, I confirmed to them to cancel the order and issue a refund instead as I had to buy elsewhere. Since then, and despite 10 subsequent emails, they've refused to issue a refund saying they're still investigating. It's now been 12 weeks! I asked for some clarity on the next steps and they're email abruptly said "We cannot".

Crappy service, I'll never buy from them again. I can't fathom it. Any suggestions?