Fritzbox LTE IPSec Tunnel for community edition 2.5.2

[flandwehr]

I have a Fritzbox LTE 6850.
dyndns is configured, ipsec tunnel is configured (several variations).
But I'm not able to bring up the tunnel.
I'm not a IPSec/VPN pro, but I need a example configuration, that is working.
I see a lot of entries in the system log like:
Aug 31 14:03:11 charon 5776 15[CFG] vici client 406 disconnected
Aug 31 14:03:11 charon 5776 15[CFG] vici client 406 requests: list-sas
Aug 31 14:03:11 charon 5776 13[CFG] vici client 406 registered for: list-sa
Aug 31 14:03:11 charon 5776 13[CFG] vici client 406 connected
Aug 31 14:03:01 charon 5776 14[CFG] vici client 405 disconnected
Aug 31 14:03:01 charon 5776 13[CFG] vici client 405 requests: list-sas
Aug 31 14:03:01 charon 5776 14[CFG] vici client 405 registered for: list-sa
Aug 31 14:03:01 charon 5776 13[CFG] vici client 405 connected
Aug 31 14:02:50 charon 5776 12[CFG] vici client 404 disconnected

but the tunnel is never established.
I tried a lot with phase1/phase2 encryption, but nothing worked.
Should it work? and how?

Kind regards for any tip.