Half Wi-Fi Speed .. [SM-N920C]

My wifi speed on my phone (note 5) is 10-15 mbps, while on every device at home, its 25-30 mbps.

And on neighbors wifi, about 30 instead of 50 mbps.

Every other device I have including GS4, iPhone 6 plus, 2 iPads all having the full speed.

I tried these:

Fully resetting router (TPlink TL-WR841N)
Forgetting network and reconnecting
Changing DNS
Using Static instead of DHCP
Clearing cache through recovery
Resetting settings
Full phone factory reset.

Anything i could try?

PS: I am not rooted, as I don't want to lose warranty just yet.

also sorry wrong section but n5 section nobody helped..