Nighthawk MR1100 LTE is SLOW when either Dualband WLAN or Dataoffloading (without connection) is ON

Nighthawk MR1100 LTE is SLOW when either Dualband WLAN or Dataoffloading (without connection) is ON

newest firmware, restarted/reset already tried, country Austria (EU), with all carriers (A1, T-Mobile, Drei) the same problem:

when either Dualband WLAN (even with only 1 client connected to 2,4GHz) or Dataoffloading (without any connection) is enabled in the router settings, LTE downstream is at approx 5 MBit/s, upstream 15 MBit/s, when Dualband WLAN OR dataoffloading is switched off (so that the router resets the WLAN unit) it levels up immediately to downstream at approx 25 MBit/s, upstream 40 MBit/s.

When Dualband WLAN (with no connection) or Dataoffloading (with no connection) is just enabled in the router settings again, LTE slows down massively again.

Please tell me if this is known behaviour or if there is a hardware fault on my device.

Router is at approx 30+ degree celsius environment.

Sometimes when it is running some days, it must also be restarted to reach full LTE speed again. Can it be set to auto-restart every 24 hours?

Bought on amazon in 2022.