@Fran910
This WR941HP is a router, it cannot be working as the modem. Please verify again if it connects to the internet correctly, ensure there is a WAN IP address. Who is your ISP, BTW? You can try to clone the main PC's MAC address to the WR941HP by following this guide:
https://www.tp-link.com/support/faq/68/
Meanwhile, ensure there is no special settings on the WR941HP, such as black list on the Access Control settings page:
https://www.tp-link.com/support/faq/1914/
It is suggested to reset the router to factory defaults by pressing the RESET button on the back panel, then reconfigure it from scratch to check if it will be working properly on all the devices. You can also follow this guide to confirm the settings on it:
https://www.tp-link.com/support/faq/1556/