@Tico25 

I do not know of any specific issue of that router that is open to easy hacking.

When you say you changed the password, did you change the WIFI passwords, the router password, the tether app password, or any of the other TP Link passwords? Any of those could be an issue.

Hackers can and do break into all brands of routers. You need to make it more difficult for them so they go elsewhere much like when you secure your home. 

You need good strong and long passwords that are not reused. Open ports from port forwarding and UPnP can allow a way to get in. If you use a DDNS service, change that user-id and password.

Your WIFI is a very common way in. If they are using WIFI to gain access. turn it off when not in use, move the router to other part of your home, if on one side of the home. block the signal to the outside, reduce the WIFI power if your router has that option, change the WIFI passwords often, ... Some router's guest networks block access to other parts of your network, so put many of those devices on it such as cameras, switches, TVs, ...

Some devices that you have in your network can be a source of concern. For example. the US government prevents some manufactures of some camera systems from being in their network.

I do not know who your ISP is but if you are using their modem/Gateway, there was some ISPs that use their gateway as a hotspot for others. If that is the case, replace it with your own modem. (in the long run you will save money).

Most routers have a way of restricting the devices that can attach, so use that feature.

Check and maybe reinstall the operating systems on you computers as they may have been hacked. Be sure they are up to date.

I do not trust all VPNs, especially the free ones.

  @ArcherC8 

Thank you very much for the detailed and thoughtful reply. 

I for sure have changed the wifi password multiple times. I am pretty sure I have changed the router password too. I probably did not change the Tether app password, but will do that and the router password too. I do not use port forwarding or a DDNS service. My passwords are are all strong, generated by my password manager.   My provider is a cable company, and their modem does not have a hotspot feature. I connect my router wired into the modem and go from there.  I do have a camera and a couple of light bulbs connected into my wireless network; could those be avenues for hacking?  I will move them into the guest network if that helps. Also will look into limiting devices that can connect if that's possible; great suggestion.

I am an IT professional and am pretty sure that my computers are safe, but will certainly check again.

I am now using a router by gl-inet but didn't want to trash my Archer router. My new router is a VPN server too, for what that's worth.

Thanks again,

Tico25