@JohnDS 

just a guess, but I suspect he has signed on to your router and changed the QoS settings.

@ArcherC8

1) Assuming that was the case, and now everything is working perfectly, do I need to change anything back?

2) How do I avoid this happening in the future besides the obvious of not letting him use it.  Would creating a guest account protect this? 

3) Can you explain the Qos settings, like what it does?

Thanks.

@JohnDS 

Go to the QoS (Quality of Service) settings and ensure he has not set his device to be the top priority device which it would do each time he signs on to the router. Setting a device to the top priority would take resources from the other devices and if you have limited amount of service, the impact would be greater. QoS is used by gamers so when they are playing their games, the games run more smoothly by taking more of the available network resources. Others uses it to balance the network resources and to give priority to specific devices such as by people working from home for the work PC. Others may lower the priority to things like printers and home automation devices. 

If he had changed the QoS, you need change it, then change the router's User ID and Password to something strong so he can not sign back on to the router. 

If anyone has had unauthorized access to my router, I would reset it to factory and reenter my settings changing the WIFI SSIDs / passwords and the router's User ID /  Password. Otherwise, I would wonder what else might have been changed such as opening ports.

I would not want anyone on my network (guest or otherwise) that has breached my security. The guest network would not provide any more protection on most routers.

Depending on the router options, you may be able to blacklist his device (you will need his MAC address) or better, white list only the devices you want to attach. White list takes more time and effort as you need to enter the MAC of all your devices. Not all routers support it. 

@ArcherC8 

There are two tabs.  One says "basic" and the other says "advanced".  The Qos tab only shows under the "basic" tab.  And after I click it, it shows this: (photo below).  Its just a black check box.  What do I do?

In general, which title setting am I looking for, for priority settings?

@JohnDS 

QoS is off which you should have unless you need it. So leave it off. If you turn it on, you will see the options for QoS.

He may be turning it on when he us there.

I would at least change the router User ID and password.

@ArcherC8 

Ok cool.  Yes I changed the password to log into the router, but not the username.  So basically when he is here, the Qos settings would be on if he were messing with priority settings?

This is off subject, but my ISP likes to somehow attach a wifi hotspot to your personal setup.  I dont like how they do this and I'm wondering if there is a way to disable this so that it doesn't slow my connection down in the case of a random person picking up the hotspot signal.  Any idea how to stop it?  Thanks.

@JohnDS 

He would need to sign on to the router to change the QoS settings. If you have the issue, sign on to the router and see if it changed.

Turning off that extra WIFI hotspot may differ based on the ISP.  I do not know how to do it. Should be able to Google it or go to the ISP's forum.