Trying to help a relative who lives far away.

He has a combination modem/router inside the house.  About 6 months ago, for a detached space that he uses as an office that had a weak signal, he purchases an EAP225-outdoor that was fastened on the outside of the building, and they connected a cable to the inside of it, to a TP-Link router, into which he connected cables to a TV and a computer. He called the Geek Squad to set it up, and all worked fine.

Yesterday, he changed ISPs. Since then he can obviously still connect to the router, but he gets a DNS error and cannot access the internet. I am just a normal guy, and not an internet expert, but I assume it is because the new unit inside uses a different IP, and that is why the AP is not connecting properly.

a) Am I correct that we have to go in and change the settings within the EAP225 and not the outside router, since the router is simply getting the signal from the EAP225?

b) He no longer has the box. How do we log in to the device to change the settings? Is the default still admin/admin if it was not changed?

c) Would it be easier to simply reset it to factory settings and start fresh, provided we know how to get it?

What other suggestion(s) would you have?

Thank you in advance.