Help Needed: RT-AC88U- Not getting correct speed on router mode

Hi Guys- I recently upgraded my internet plan from 100mbps to 300 mbps. The internet provider changed the ISP supplied modem which now has dual band unlike before that it's only single band.

Here is where I encounter some issues. When I turned off the wifi for the ISP router and used my asus router it is only reflecting 45mbps. It doesn't happen before with my old plan. When I used my Asus router as an access point mode then it will show the correct speed.

I'm wondering what I did wrong...

Help Needed: RT-AC88U- Not getting correct speed on router mode