@Juzcat 

As promised, steps to try.

Take a note (write down) your TP-Link router MAC address. You may need this info later.

Without changing wiring, configure Deco mesh to run in Router mode. You will have Main Deco in Router mode still connected to TP-Link router. Verify that in this configuration Deco mesh connects to Internet and is healthy.

Power off modem, TP-Link router, Main Deco and all Satellite Deco. Keep Satellite Deco powered off till the end of this exercise, you won't need them. Leave everything powered off for 3-5 minutes, then proceed with next step.

Unplug TP-Link router from Ethernet cable that connects it to modem and plug Main Deco to that Ethernet cable. Power on modem and wait until it is online, then wait another 3-5 minutes. Power on Main Deco. Wait additional 3-5 minutes. If Main Deco can't get Internet connection, proceed with next step.

Reboot modem while keeping Main Deco on. Wait 3-5 minutes and see if Main Deco connects to Internet. If not, reboot Main Deco while keeping modem on. Wait 3-5 minutes and see if Main Deco connects to Internet.

If still no success, you will need to configure Main Deco with TP-Link Archer router MAC address, the process known as MAC address cloning. If you can't access Deco app configuration to do that, restore original wiring configuration: modem<->TP-Link router <-> Man Deco.

After you've updated MAC address for Main Deco and connected it back to modem, if still no Internet access: reboot modem, reboot Main Deco, see if that does the trick.

------------

If you got to this point with no success, I have no other suggestions. Something is not working (obviously), but I can't help remotely with that. You may wish to contact your ISp Tech support with the question why replacing one TP-Link router (Archer) with another TP-Link router (Main Deco) does not work and if they have any suggestions on what could be wrong.