ZyXEL NR7101: modem gets IPv6 prefix and name servers but internet access fails

Hi alltogether,

for me it's the first time getting in touch with openwrt and with this community but not the first time doing networking.

I have a ZyXEL NR7101 running but can't get it talking to the internet.
I successfully followed instructions from wiki:

I get reasonable results described in section "Interface configuration":

after doing all steps mentioned above the last command shows:

~# uqmi -d /dev/cdc-wdm0 --get-data-status

If I restart the wan device in LUCI interface the status changes to connected

But I cannot a access the internet.

The wwan0 device gets a public IPv6 also does my via br-lan connected dhcp/IPv6 capable client. Also the providers DNS are found in the logs and in the resolver config:

~# cat /tmp/resolv.conf.d/resolv.conf.auto 
# Interface wan
nameserver 2a01:598:7ff:0:10:74:210:210
nameserver 2a01:598:7ff:0:10:74:210:211

Pinging an known foreign IPv6 ends with timeout.

After a while - some 5 minutes - the status silently switches back to disconnected not showing anything in the logs.

this is the wan configuration:

config interface 'wan'
	option proto 'qmi'
	option device '/dev/cdc-wdm0'
	option auth 'none'
	option pdptype 'ipv4v6'
	option apn 'internet'

Any ideas how to debug this?

RTFM happily appreciated ;o))