OK, please try this configuration:
config interface 'lte'
option proto 'qmi'
option device '/dev/cdc-wdm0'
option apn 'internet.v6.telekom'
auth 'both'
username 'telekom'
password 'tm'
option ipv6 '1'
option pdptype 'ipv6'
option peerdns '0'
list dns '2001:4860:4860::64'
You don't need clat/464xlat interface at this point.
Then reboot and try nslookup and ping against ipv4.tlund.se
To be on a safe side the APN needs to be adjusted on the modem itself, see my recommendations from TP-Link TL-MR6400 v5 - LTE but no internet connection - #14 by AndrewZ and below.