need some help from experts here regarding router setting...

my setup :

- BTU : Alcatel Lucent I240G
- Router : Dlink DIR-842 (NEW)
- Cable : CAT 6
- On Unifi 500Mbps

i follow the setup wizard in the dlink router to setup my unifi for both 2.4Ghz and 5Ghz...

i use my phone to connect to 5Ghz wifi and speedtest can reach around 200mbps, which is ok for me...then i use my laptop to connect to the 2.4Ghz wifi (my laptop does not support 802.11ac), the speedtest result is disappointing in which the result only give me 25mbps for both download and upload speed...

before this i only use my old router which is the TP Link TL-WR841HP Wireless N router, and my laptop (connect to 2.4Ghz wifi) can get constantly good speed at around 75 to 80mbps for both download and upload speed, upon change to the dlink dir-842, the speed drops a lot and never can exceed 30mbps...

have i done something wrong on the dlink 842 router setting? or i need to do extra setting for the 2.4Ghz wifi?

really appreciate some help here...thx a lot...