LTE3302-M432: Problems in local network, can't reach devices, reconnects

Hello,

I have problems in my local network where LTE3302-M432 is my router.

Occasional symptoms:

- Devices like Raspberry Pi or Smart TV lose WiFi connections within seconds (didn't happen last days, now it's fine)

- Can't connect from PC to these devices although I can see them connected in router's DHCP or ARP table. Sometimes it's working after x times rebooting, resetting within several days, but I can't wait until I can use my devices now!

- Fluctuating download and upload speed

The current problem is that I bought a Shelly Smart Plug, which has got the IP address 192.168.1.42 and I can reach it in my PC browser but not from my smartphones and the ESP. The PC is connected through a switch to the router. Other devices are connected via WiFi. As I said, I have this problem very often. It's as if the router has to get used to the new devices.

My router has got the newest firmware, I already reset it (I don't want to do this every week). The shelly plug has got a fixed IP in the router (Configuration > DHCP Server > Advanced > Static DHCP Table).

What's the problem? Can I provide more information to debug this?

Thanks.