@Briston1969 Adding a bit more context here. I have been following, or trying to follow, the instructions below (substituting my ISP's reported VLAN ID - 101 - for the one mentioned there).

https://superuser.com/questions/1148268/how-to-ditch-centurylink-fiber-modem

One of the things I find worrying is that I cannot access the web GUI when my laptop is connected to port 5, but ports 1-4 are fine. This is not an issue, though I can't find any reference in any literature as to why this might be the case or any references to the ports having different qualities. I thought for a moment it might be a PoE port, but I don't see any reference to the SG105E having this feature.

@Briston1969 

Hi there, after you configure the switch, please make sure that there has no other devices connecting to the switch except the router. If there has another device connecting behind the switch, the router will have no internet cause the modem's WAN IP can only provide 1 device with internet.

Here is my advice: we consider port 1 on switch connecting to the modem, and port 2 on switch connecting to Google mesh unit.

For port 1, set up VLAN 101, tagged port, PVID can put in 1 cause it will not affect.

For port 2, set up VLAN 101, untagged port, PVID 101

The tagged can maintain the internet data with VLAN ID, the untagged means "remove" tag.

Wish this can help you and want to get your feedback.

Have a nice day!

@John1234 Thanks so much for your advice. Unfortunately I have been unable to insert the switch between my bridged router and the Google Nest Wifi without losing internet access. Possible explanations include:

I have the wrong VLAN ID for my ISP (BT - big UK ISP - VDSL is reported to be 101, but the ISP itself refuses to confirm)

I have been unable to follow the instructions exactly - I have had to use the TP Link configuration utility rather than the web GUI (connecting to the switch alone and typing in 192.168.0.1 does not bring it up) - and I do not get an option to save config. I have not been able to set separate PVIDs for ports 1 and 2

There is no VLAN tagging issue. I have used various packet sniffers on various ports and have yet to see any evidence of VLAN tagging on it.

So, I am still stuck unfortunately. I will see if one out of my ISP and Google moves from their unhelpful position, but I am very grateful for your help