Fritzbox 7490 Wi-Fi problems?

Jo got 100 Mbit download and 40 Mbit upload from Telekom.

So with me connected to the PC with Lan Netflix etc. Works without problems

but with my sister connected to Fire TV Stick and Wi-Fi, the picture sometimes hangs and it loads so long but only sometimes. Sometimes it works without it. Can you do something about it? And no matter how you use my cell phone, I still have full bars.

Maybe there are different workloads -

nothing to do.

I suspect that you have the latest firmware on the 7490?

We had the 7490 in use here for 3 years without any problems, even on a 200/100 FTTH connection (fed in via LAN1, because the DSL port of the 7490 goes up to 100 at best - but that shouldn't have anything to do with your problem anyway) - and several video streams could run simultaneously via WLAN.

Since the last firmware update, however, it has been bothering regularly - you noticed that even the loading times of websites were sometimes significantly longer and of course there were problems with video streams. The whole thing was not always like this, but in between it ran again and again - we could not see a pattern.
Whenever the problems occurred, the ping times to the box also shot up again and again tremendously (clearly in the 2 and sometimes even 3-digit range, where they should actually be well below 1).

We happened to have a 7590 standing around there and then just swapped it, but if we hadn't had it, I would have tried first to see if I could downgrade the 7490 to the penultimate firmware version (I think it was v7.11), because this one Problems clearly started with the upgrade to firmware version 7.21.

But of course I can't say whether that's the cause with you. If the problem with the previous firmware version already existed or you currently don't have v7.21 on it anyway, it will probably be something different but otherwise and especially if the problem is not consistently the same, it could be be good!

Incidentally, I would not only look at the number of lines for the reception quality but also do a speed test there and measure ping times if necessary.
Do you use the 2.4GHz Wi-Fi or the 5GHz?