RAX40 lose connection to internet

I bought a netgear rax 40 last september and if a single wall is between the wifi and device, the connection to wifi is steady but connection to internet is lost. It didn't happen when i was using R6220. Also, internet is frequently just stop. I don't know why this happened so i tried various method. 1. i abled/disabled wifi AX mode,beamforming,160mhz,...etc 2. Call for telecom(or isp) and check the modem 3.downgrade/upgrade the firmware but it just keeps happened. I have about 14 devices connected to my RAX40. Does anyone have solution about this? I wonder how to fix this problem or just toss away RAX40.