I have 300mbps fibre broadband, which is way better than our old 30mb!

I have my pc connected to the internet via a powerline adapter and ethernet cable and did a speed test. The speed test says I’m getting 57mbps download and 57 mbps upload, well below the 300mbps advertised.

When I run a speed test over wifi on my phone I get approx 100mbps download and 85mbps upload. The wifi is over a TP-Link Deco mesh.

I never expected to get the full 300mbps, but what is holding me back from getting closer to the advertised speeds?

Is it the powerline adapter being too old (approx 4-5 years), the mesh wifi, the ethernet cables being limited to 57mbps, or something else entirely?

I’m reasonably tech savvy but internet / home connectivity etc is one of my weak points!

Any suggestions for things to try / things to upgrade or do differently would be great! Thanks in advance!