First, can you tell us a little about the setup? What is being used for a router? Is there a single wi-fi access point or multiple? What kind of space? Additionally, can you and the renter be there at the same time to troubleshoot?

Second, we’ll want to determine whether the issue is signal strenght or the device. I suggest something like the following. I’m going to assume that the tenant has a cellphone in addition to the PC.

1. You and tenant each install WiFi Analyzer or WiFiman or similar app on your phones.
2. You and tenant each install Speedtest locally on your phones and on the tenant’s PC (if allowed).
3. Both of you check w-fi levels at the PC location. If the signal is less than -60 dBm ( say -70) then start with the assumption that the signal is too low to be reliable. If it meets this criteria, go to the next step (indications are the signal is good).
4. Run Speedtest from the phones, and then from the PC. Do they show similar speeds? If the PC is much slower, investigate the age, and check the NIC card specs. Old NIC’s may be 10 mBps, some will be 10/100, and a most that are 5 years old or less will probably be 10/100/1000. (What kind of speed does your ISP plan support? Are you getting it?) 
5. By this point, you’ll have an indication of the constraint. If it is only the PC that has an issue, that helps narrow the problem. If everything looks good, check whether the laptop is consistently used from the same location. This is especially true if there are multiple routers/access points. Some devices will connect to one point and not release when you move the device to connect to a stronger point, without manually dropping and restarting the connection.

Hopefully that gets you started. Report back if we can assist further.

This is a modem/router in one. I went to the house to try and assist with the problem. I was able to connect easily with my cell phone, and speed test showed reasonable upload and download speeds. Her work iPhone was also able to connect with similar speeds, but her personal iPhone and the work laptop could either not connect, or had speeds of like 1 Mbps. Not nearly enough to attend a Skype meeting, which is what she needed to do. 

I missed the reference to SBG6700AC earlier. Certainly more than capable for the purpose. I’d still start with checking the signal strenth and speed in close proximity to the device, and then at the desired user location.