Strange routing issue, may be related to VPN

[slovx2]

My Environment

Oneplus9 pro Android 14
Unlock root. Run Ubuntu in super administrator mode, ssh remote login.

Problem Brief

After using AnLinux + termux to install Ubuntu, network anomalies occurred after a while. However, everything is normal outside the Ubuntu system (including termux and the Android system).
Connecting to any VPN (enabled on the Android system) resolves the issue. However, the network access speed is very slow.
The problem reappeared after turning off the VPN again.

Specific description of the problem

Installed Ubuntu using AnLinux + Termux.

In the Ubuntu system
Everything was normal at first, but after a while, strange routing issues would occur.
Specifically, it manifests as an error when using curl to access any address: Could not resolve host xxx

For example

curl www.baidu.com
Could not resolve host www.baidu.com

It can be determined that it is not related to a specific domain name, as accessing any domain name is the same.

Connecting to any VPN on the Android system will restore normal access, but the access speed is noticeably slow

Problem Troubleshooting

Install iproute2.
Comparison of IP router with and without VPN

apt install iproute2

When there is a VPN

root@localhost:~# ip route
26.26.26.0/30 dev tun0 proto kernel scope link src 26.26.26.1
192.168.31.0/24 dev wlan0 proto kernel scope link src 192.168.31.230

Without VPN

root@localhost:~# ip route
192.168.31.0/24 dev wlan0 proto kernel scope link src 192.168.31.230

Based on this result, ChatGPT suggests that I execute

ip route add default via 192.168.31.1

The IP 192.168.31.1 is my home router.

The problem has been solved.

Problem Impact

Although ip route add default via 192.168.31.1 can solve the problem, I cannot keep adding this route manually.

How else can I continue to investigate this issue?