Asus RT-AC87U Dual-band Wireless-AC2400 problem with AP mode

[unrealbe]

OK so I use this router for two things:
1) Get wifi upstairs
2) connect both my server and my home computer to my modem/router downstairs

so I put him in AP modus.
Operation Mode:Access Point(AP) mode Firmware Version:3.0.0.4.380_3459

Everything works OK but when I want to stream contect from my homeserver to my tv using plex I get a lot of stuttering. Only when I downgrade the stream to like 4 mb/s I get a watchable picture. If I remove the router and just connect the server directly to my internet cable I can play just fine on plex.

Anyone has any idea what can help me? Some setting I don't know about or maybe other firmware that can do bridge mode better?

I'll try and call asus support Belgium on monday but I highly doubt they can help me with this issue.
Thank you

 

[unrealbe]

Edit:
I tried in router mode and I had the same issues. (using subnet x.x.2.x while my other router was on x.x.1.x)