So, disabling IPv6 solved the loop issue:
nmcli con mod BRA-BrTCelular-d8a338232cefff166e10be70b28cdf24371041dc ipv6.method "disabled"
But its a shame I had to do this, since @aleksander0m mentioned newer versions of ModemManager has this fixed. Is there any kind of release cycle? He asked it here Which versions of ModemManager and NetworkManager does BalenaOS runs? - #16 by aleksander0m but didn’t get any response.