EX2700 slow download speed. Should I upgrade?

Hello there.

I think my EX2700 extender's speed has gone down recently.

I used to be able to get a decent speed of around 10 Mbps, now it's quite unstable.
Sometimes I even get down to 3 Mbps.

However, if I connect to my Wi-Fi directly to the main router (in the same place in my room as I connected to the extender), the speed is good but the ping takes somewhat longer.

I'm not sure if this has to do with my neighbour's Wi-Fi upstairs.
I have already changed my channel to be as far away as all the other occupied channels as possible (Their channel is 3 and mine is 13).

Would upgrading my extender to a dual band AC750 (like the EX3700) help with the situation?

The advertised speed from my current ISP is 19Mbps. I am switching to another ISP with 57Mbps soon so I am also wondering if I should get a better extender to catch up with the faster speed.