@Harry13 

"TP-Link recommends 'Router' mode" - you can ignore that recommendation. If everything works with Deco in Access Point mode, keep running it in Access Point mode. This is what I do with my Deco mesh that has M5s in it, since I got them 2 years ago.

  @Alexandre. Yeah, perhaps "If is aint broke, don't fix it" should be applied here. I am now just curious. I was originally worried about problems reported on internet forums with reassignment of IP addresses in Access Point Mode, but I disabled the 'smart DHCP' feature and hope that will stop that happening.

There should be a specific layman's guide to setting up TP Link Deco M5 with TalkTalk, but I could not find any on the internet from TP Link, TalkTalk or anyone else. It's a good job I know a little about networking: I don't think a layman would have been able to set this up despite the very good TP Link app and a not bad TalkTalk web interface.

  @Harry13 

If you are familiar with networking, try this approach. Use laptop or desktop with Ethernet port. Try to configure TalkTalk router to run in bridge (modem) mode. If TalkTalk configuration is successful, you can connect laptop/PC to TalkTalk Ethernet port and have Internet connection.

If not, contact your ISP Tech Support. Do not mention Deco, or your escalation will be dismissed because of "third party hardware/software, not our problem."

Tell them you use laptop/PC to verify that TalkTalk is successfully configured to run in bridge/modem mode. Ask them how to setup TalkTalk in bridge/modem mode and if it is possible.

If they ask why would you want that configuration, tell them you are considering third party router and want to see first if TalkTalk can be configured to run as modem only.

After you've managed, with the help of your ISP Tech support, to switch TalkTalk to bridge/modem mode, and verified with laptop/PC that it works, you can just connect Main Deco to it in router mode and everything should work.

If your laptop/PC can get Internet access through the modem, so will Main Deco.

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Another option is to run Main Deco in router mode, connected to TalkTalk also running as router, and make Main Deco a DMZ host in your TalkTalk router settings.

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Finally, there is no harm to run Main Deco as router, connected to TalkTalk running as router. If you do not have apps (VPN usually) or hardware (such as XBox) which complain of double NAT and refuse to work, that setup is quite OK. If you do, you'll know right away.

It also adds layer of security: to get into your home network from outside hacker will have to penetrate TalkTalk firewall and then penetrate Main Deco firewall. Too much work for most hackers.

  @Alexandre. I tried all of that (as I think I said). I had no problem setting TalkTalk into bridging mode, but it did not solve the problem. There are also separate TalkTalk switches to turn off NAT and DHCP. None of those combinations worked as long as the Deco was in router mode. When the Deco was in Access Point mode it did not seem to make much difference; then most combinations did work.

Maybe I missed a switch; or maybe there is one specific combination of the switches that I missed ... ?

  @Harry13 

If you successfully set Talk Talk Router to bridge mode, you should be able to connect laptop or desktop to it with Ethernet cable and have Internet access. Have you tried that?

Also, when you did initial configuration of Deco mesh, Main Deco starts in router mode, which means you either configured Deco with different router or at the beginning you had Deco in router mode working with Talk Talk Router.

How did you do initial configuration of Deco mesh, when you bought it?

  @Harry13

to Q1. No because I do not have a laptop with an Ethernet port. Why do you ask? I had no problem connecting my laptop to the router using the old (router) WiFi network, now I can connect to it using the new WiFi network, so I do not see how it bears on the problem?

to Q2. I initially set up all three Decos with the Deco app using the default router mode (not even knowing that was the case at the time).  The app said everything was fine, but I did not have access from any device connected to the new network, either to the internet or other local devices. I changed the router settings by (a lot of) trial and error (see my original post) but could not get it to work until I hit upon changing the Deco to access mode. Funnily enough, having done that, putting the TalkTalk router in bridge mode or not makes remarkably little difference, it works either way! I have a hunch that the problem has something to do with DHCP or NAT (or both) -- these are separate switches in TalkTalk -- but I think I tried all the combinations without success.

BTW, I now disabled the old network and the new one is working in access mode, so it is just a matter of curiosity for me now (a) why was it so difficult / why are there no clear instructions anywhere? and (b) why does the TP-Link Deco M5 in 'router mode' not work with Talk Talk Router in bridge mode / are there other router settings that would allow it to work?