The thing is.. speed will drop after quite some time
If I reset/reconnect.. then speed will be max again...and after a while.. it'll become unstable

Current test on 4 devices (2 phones, 2 tablets) ... I got around 15mbps down, 3mbps up..
Probably due to weekend

My take on using B618 for congested area... the short answer is no...

I've been hopping modems/router since last year..
from K5005 (USB modem) > AC760 > AC790s > B618
You'll see improvement (even in speed) when upgrading.. but it won't last long..
A few weeks later.. you'll see drops in connectivity (speed and stability)

I used to have 15-20mbps on AC790s when I first bought it...
But steadily declined to 1-5mbps in weeks.. and dropped connection A LOT..

Switched to B618..
I got 25-40mbps at start.. and then i goes back to ~10mbps a few weeks later..
And now.. I mostly got 1-4mbps at night (8pm - 1am)... And disconnection still happens..

You MIGHT have different scenario tho..