@Stewart From the screenshots you have various ad blocker(s), configured for the browser. You could try disabling the add-ons temporarily to see if that helps, or whitelist the administrator portal...

@Brix - It does have a modem, sorry I gave wrong info. Maybe the emulator might help you to find out which parts need filling in, since you already have some login details....

@Brix You could put your original Asus back in, then connect a network cable from the back of the Asus origin to the blue port of the Archer wifi router (it might need configuring). Existing modem...

@Brix Have you tried accessing the administration console of the Archer device via PC/Laptop web browser, rather than a mobile device? There are probably more items that need configuring that you are...

@ksaefos You could try configuring the QoS settings on the modem, if you haven't already? I doubt very much TP-Link will release anymore firmware updates for this model and version.

@ksaefos You could try the various firmware updates as advertised for this model here. https://www.tp-link.com/uk/support/download/archer-vr900/#Firmware