Set the timestamp as appropriate when importing, or force a rescan after importing with rescanblockchain. Either way, once a rescan completes, any historical transactions will be discovered and included.
Thank you achow101 for your feedback. I checked the date of my first transaction and choose a date 3 months before that, then transferred it to unixtime and used that date for the command, I only imported the wpkh descriptors. At first I was surprised that the legacy transactions didn't show up in the history. The I checked for the receiving addresses and change addresses of the descriptors and found out, that these are new addresses - so I assumed that the legacy transactions must be inside the combo descriptors. But unfortunately the import of the combo descriptors were not successful, can't remember the error code but I assume I made a mistake with the command syntax. For the average user it is not easy to accomplish this task - but I'm learning
