WLAN on the iphone crazy?

Am
2

For a few weeks now, the Wi-Fi on my iPhone XS has apparently been crazy. When I try to open a website via Safari, nothing happens forever! If I then switch off Wi-Fi, the page loads immediately via LTE. My other devices (laptop, smart TV, Alexa) at home work great. My cell phone also takes forever to use Wi-Fi at work. Do you have any idea what the reason could be? So far I have already freed up memory and deleted the Safari data and of course restarted my iphone and router.
Maybe you have an idea? I'm slowly reaching the limits of my data volume!

Sp

There's such a special restart with hold down the power button and hold down the volume minus until the apple appears, have you tried this yet? - unfortunately do not know what the restart is called exactly.

Am

I've already tried that. Unfortunately, it still doesn't help.