So I found a bug, that was already in place in 5.0.4 ( → link)
I did setup my TOS (Indiegogo-Version, 2GB RAM) after reflashing via 4-LED-method as LTE-Router and installed the following packages:
kmod-usb-net-qmi-wwan luci-proto-qmi uqmi sqm-scripts ca-certificates luci-app-ddns wget kmod-sched-cake iperf luci-ssl tor tor-geoip luci-app-wireguard wireguard qrencode block-mount kmod-usb-storage-extras kmod-fs-ext4 e2fsprogs speedtest-netperf
After some reboots syslog is not available anymore:
I am right now generating diagnostics and send them to turris support.