Wifi gets too slow after some minutes

[wifu]

Since I've tried any method I could find online, I decided to get help here.

I shut off wifi optimization, rebootet my phone, my wifi is certificated and the wifi also gets slower un safe mode. The first three minutes after I reboot my phone is the only moment when I can connect to any wifi. The strange thing is, that this started just about a week or so after I've bought the phone. Meaning that I had no problems at the beginning and could install all my apps I had on my previous android device.
But since the problem still exists in safe mode, I can't understand why it is the case with the wifi.

I'm glad for every help.
Thank you

[Uliwooly]

Do you have auto backup on G+ ?

how about auto update apps while on wifi?

[wifu]

I don't use G+ and it seems that the auto backup is turned off as well.
About the auto updates, if you mean the google play settings, I have auto update only with wifi.
Does this affect my wifi connection?
And additionally, I tried the "solution" with the apps reset of another forum. The wifi then stayed a bit longer than usual (about 10min).

[Uliwooly]

yes it does, bc it could be updating mutliple apps, slowing down your connection. 

[wifu]

I see. Thanks for your advice
I always had this setting on my previous devices (Sony Xperia X10 pro, Samsung Galaxy Slll) and thought it wouldn't do anything. It just got slow when it actually was updating.
Now I turned that setting off, the wifi worked quite long again (10min) but then it was gone. Unfortunately.

[wifu]

I wanted to add that the phone stopped connecting to the wifi completely.
It shows that it is connected and it haa got a very good connection but it does not.
I can turn off wifi and turn it on again this trick doesn't work anymore.

[Uliwooly]

download speed test 

[wifu]

Ookla speedtest results:
Download: 33.70Mbps
Upload: 14.35Mbps
Ping: 24

[Uliwooly]

Does it work fine with other routers?

[wifu]

Unfortunately not.
But I downloaded an app called wifi fixer and wifi works again. I just don't know for how long...