Hi all!
Recently I've set up my home network using Mikrotik RB 750Gr3. LAN works fine, configuration seems to be ok:

My network settings:
Router: 192.168.1.1
Netmask 255.255.255.0 / 24
DHCP server set at range: 192.168.1.10-254
Router gets IP address from ISP automatically- DHCP.

To achieve high transfer speeds at wireless network I've purchased AP-only Asus EA-AC87. Set it to static IP address 192.168.1.2, pointed router at 192.168.1.1. Wifi works.
The problem is that my AP reboots itself randomly (but it is easier to replicate when I run speedtest.net). To exclude fault of my AP hardware I've also tested it with oldish TP-Link AP/router with DHCP off, which behaves the same. RouterBoard does not reboot, works solid stable. I've also did tests on few ethernet ports to exclude fault of ethernet wires.
While Asus AP reboots I get a placeholder website saying that it is currently rebooting because its IP address has changed recently.

I've ran out of ideas. Maybe you can help me?