Trying to setup old Huawei HG630b as an AP

I must be missing something simple. I'm trying to setup an old Huawei wireless router as a 2nd access point by following the instructions from @Jase2985 in this old post here, but I'm having problems.

https://www.geekzone.co.nz/forums.asp?forumid=66&topicid=173220#1305877

http://www.practicallynetworked.com/networking/convert_wifi_router_to_access_point.htm

Basically I'm falling at the first hurdle. Note my current router (an Asus RT-AC58U) has a DHCP IP pool within 192.168.1.2 to 192.168.1.254. I can login into the admin page for the HG630b, and type an alternate static IP (I've tried 192.168.1.252) but when I hit Submit to commit the settings, the HG630 basically becomes unresponsive and I have to do a factory reset to be able to see the admin page again.

Seems like there is some kind of conflict going on - but I'm no expert at these types of issues. Note the IP address of the HG630b is 192.168.1.254.

Any ideas most welcome.