I found a solution. Searching for the device id of my phone (05c6:9024, from lsusb) led me to an OpenWRT discussion: UF896 - Qualcomm MSM8916 LTE router ~384MiB RAM/2.4GiB flash, Android: OpenWrt? - For Developers - OpenWrt Forum
The solution is to edit /etc/usb-mode.json
and remove these lines:
"05c6:9024": {
"*": {
"t_vendor": 1478,
"t_product": [ 36901 ],
"mode": "StandardEject",
"msg": [ ]
}
},
After that I plugged my device again and I can tether without any issues.