the issue is resolved. it looks the modem is bundled with original router's MAC address for some time. someone said for M1, the time is 10 minutes.(maybe some time in some early years, may not be the...

just bought a AX73 a few days ago, tried to replace my old asus RT-AC1200G+ ISP: M1 fibre modem: huawei HG8240H -- this is a very old modem. now everything looks fine, but just don't have internet....