RT-AC3200 - admin GUI timeouts

[Al__]

Got new RT-AC3200, from legacy I have ~15 devices with static IPs on 192.168.100.xx with gateway configured as 192.168.100.2 so I changed the new Asus router to same IP 192.168.100.2 with 255.255.255.0 mask (that's how I had ~5 routers over last 10y). All works fine, static IPs reservation table, DHCP, routing, I can browse the internet from all clients. But the big problem is when I try to access admin GUI http://router.asus.com or http://192.168.100.2 it freezes. The browser just spins for long time then timeouts. Doesn't matter wireless or wired computer, IE or Firefox. I can ping the router (either by name router.asus.com or IP), I can telnet to it on port 80, even Asus discovery tool finds it as 192.168.100.2 but attempts to access the admin GUI timeouts. I noticed that if I power cycle the router I can access the built-in Web server, play with menus but after few minutes the GUI freezes again - until next power cycle. Pity with new design of power button (push button not the switch) I can't powercycle the router remotely through UPS socket, so have to run to the cabinet every 3min. Any ideas/help?? Tried to contact ASUS - no help - usual advice "use our discovery tool"/"make sure your laptop has correct IP address"...