@VanSquished Have you tried http://tplinkmodem.net? I think the PC browser might remember the old entry. You could also clear the browsing history and try again.

@MehdiMomeni As far as I know, TD-W9960 does not support firmware recovery via TFTP.

@wulfstan Based on my experience with DSL before updating to Fibre, service, It is always good to keep the original ISP-provided modem though it was not the latest but stable enough. Then you could...

@VanSquished I think it would be the IP settings on the main desktop and maybe you have set up a static IP address which still belongs to the old modem router: How to find IP address on your devices...

@Jack4614 You don't need to access network settings to add one more Deco. It could be added under Access Point mode directly: https://www.tp-link.com/en/support/faq/1590/

@diegolaz Are you able to connect the 5th Deco via Ethernet cable to the nearest unit? Deco all use undetached antennas and there seems to be no way to add an external antenna. How far it is from the...