the CLI device on a thread network can only sometimes communicate with internet

I have set up the OTBR with a RPi 5 and an nRF 52840 DK as an RCP. The CLI device can join the thread network as a child, the OTBR as the leader.(https://www.youtube.com/watch?v=xCyzNzYSG4U&t=1931s). Everything is ok except the last step: ot ping 8.8.8.8 from my CLI device. it can ping 8.8.8.8 successfully 1 in 10.
the question is I can ping IPv4 like 8.8.8.8 or the IPv4 of my phone (in the same LAN) with sudo ot-ctl ping 8.8.8.8 on RPi,
but in the CLI device (nRF5340 DK), it can only ping 8.8.8.8 sometimes; most times it fails. But sometimes it can ping 8.8.8.8 successfully and receive the return packet. What should I do?
I have installed the newest version of RPi from the official website(https://www.raspberrypi.com/software/). System: 32-bit Kernel version: 6.12 Debian version: 12 (bookworm)

Does anyone know what is happening? Thank you in advance for your help!

Best regards,
Jinxiu

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