No internet via main Wi-Fi but via guest Wi-Fi. To do something? Fritzbox?

I have had a problem with my Fritzbox Cable 6591 for 2-3 months.

The problem is that whenever I connect to the WLAN and leave the room where the router is, I have full bars but no access to the Internet. The funny thing is that when I'm in my "guest Wi-Fi" (connected) it doesn't matter where I'm in the house because I always have internet access.

Both accesses transmit in 2.4 and 5 GHz.

The support from AVM does not find a solution and can therefore not help me.

My provider (Vodafone) sent a technician here and he changed the box, but unfortunately without success.

Resetting the router and resetting all smartphones, laptops, and smart TVs were also unsuccessful.

Do you have any ideas?

Or did you have a similar problem and know what to do?

I'm grateful for every imaginable solution.

Below is a picture of the problem (taken from a Xiaomi Mi 10)

First, I would look under "Internet" => "Filter" => "Access Profiles" in the "Standard" profile to see whether access to the Internet is granted.

Then look further down on the page under "Assigned network devices" to see whether the "Standard" profile is also used for the device used.

The support from AVM does not find a solution and can therefore not help me.

If the support did not carry out the above-mentioned check with you, then this employee should be fired. 🤷♂️

Resetting the router and resetting all smartphones, laptops, and smart TVs were also unsuccessful.

It is clear that it does not help - that is blind actionism. 🤷♂️

Thanks for the answer. I'll try it out right away.