wi fi - LG G5 Slowing down the house internet

The story started about a month and a half ago. At random times the internet speed went down drastically, changing between 0MB/s to 75MB/s (which is my maximum).

I have called countless times to my internet provider/supplier. They claimed they supply the maximum amount I paid for. In 3 weeks we replaced about 3 different routers and the issue did not disappear.

I have heard that video games, videos and some programs on the computer can cause those issues. In my house there are 5 computers, and this issue never occured until now. I ran virus scans on every PC, checked for torrent programs, but everything was completely fine with the computers.

In one of the calls with the internet provider, one of the workers told us about a special case where his phone did the same thing we described.

My family and I started investigating more into it and we found out the LG G5 my sister owns is affecting the internet as long as it's connected to our WiFi.

The internet "slow-down" happens at random times, but it always happend when the phone was in the house and connected to the WiFi. Each time it happened as soon as we disconnected it from the WiFi the internet speed jumped straight up to 75MB/s. I can verify that it is indeed the phone and not a PC or any other device that is connected to the internet.

We have tried "factory resetting" but the issue kept occuring.

I'll list the applications and phone information down below:

Hardware Info

Model Number: LG-H850

Software Info

Android Version: 6.0.1

Android Security Patch Level: 2016-04-01

Build Number: MMB29M

Software Version: V10d-ISR-XX

Applications

Best phone ringtones

ai.type Emoji Plugin

ai.type Electric Glass Theme

ai.type Keyboard Plus

ai.type Hebrew Dictionary

Instagram

Tiny Flashlight + LED

To-do list & Agenda by Any.do

The rest of the applications came with the phone.

Now... I am desperate, very very desperate. I have been digging the internet to try and find a connection between a mere phone to the internet. This issue didn't not occur at the moment we brought the phone into the house, which makes it even harder to find a solution.

My question is: Is there any solution to fixing that internet slow-down other than throwing the phone out of the window?

Thank you in advance.