Connected no internet... But only off the primary router

[#1]

Weird issue. I've had a Fritz Box router for years. It periodically says that the DSL connection is fine but a web service cannot be contacted... And as long as that's the status then DHCP clients on the router cannot get to the internet.

But weirdly one of the DHCP clients is another router and anything connected to THAT router says "no problems here guv!" - the wired and wireless clients on THAT router don't drop network connection.

The only thing I can think of is that the second router is completely ignoring the results of a web service check possibly related to DNS, while the Fritz Box does the check and it fails.

A trace route from a PC on the second router shows the name resolution goes from the 2nd router to the Fritz Box, then to the Plusnet servers. Connect same PC directly to the Fritz Box and it gets as far as the address the DSL connection gets and then fails to get any further.

[#2]

So, let me start by making sure I understand your setup: no plusnet router, instead you use a FritzBox, connected to your phone line. 

So, on the WAN side, it should be picking up a WAN IP address from the network, plus DNS servers (alternatively, they may be manually configured)

On the LAN side, the FritzBox will be a DHCP server, providing an IP, plus the details for the default gateway (itself), and the DNS to use (either itself, the learned WAN DNS details, or a static setting - eg to 8.8.8.8).

It is quite possible that the problem is with DNS: a couple of months ago, the TWO DNS servers that my plusnet router had auto-learnt on the WAN were offline, and as pretty well everything on my LAN just used the router as a "relay", I was dead. I went onto the router, and set up static DNS servers (pointing at 8.8.8.8 and 8.8.4.4... I think!) and that fixed the issue.

Sounds like you may need to stop your FritzBox from being quite so clever!

[#3]

I thought that but no. Even if I set up a static IP address on the Fritz!Box PC client with Google or Cloudflare DNS it still loses connection.

Unplug it from the Fritz!Box, connect it to the other router, switch it back to DHCP, and bingo. Connected.

The second router sends its DNS traffic to the Fritz!Box. It isn't using public DNS servers.
Nothing has changed on the Fritz!Box config in two years. It had a firmware update in the autumn but the problem has only recently started.
The diags on the Fritz!Box are saying the ISP line is connected but it can't talk to a web service.

[#4]

And as if by magic it's just come back up on the Fritz!Box clients... With no DNS or Plusnet IP address change. The router's saying that service it couldn't reach is back up

[#5]

As I suggested, sounds like your "clever" FritzBox router is either (a) Too clever for it's own good; or (b) not clever enough!!

Any particular reason that you use the FritzBox rather than the provided PlusNet router (apart from "I paid for it so I'll damn well use it!") 

[#6]

Combination of reasons. I was with Metronet ~22 years ago and came to Plusnet through the acquisition. So I didn't get a router that way.

We did get a PN router donkeys years ago, but it simply didn't have enough wifi oomph to even cover INSIDE the houses we've lived in let alone outside. So I've always had an aftermarket option.

AVM came very highly recommended by my German colleagues, who were doing seriously fun stuff like getting ADSL working over ISDN lines. Over 20 years ago they sent me an AVM PCI ADSL modem and it worked a heck of a lot better than other routers I tried - but of course it was limited to being used by one PC.

I had a Billion Bipac for a while, and the modem wasn't quite as good as the Fritz! card but the power of the wifi was amazing.

The two houses we rented before were 3 storeys (4 if you count cellars). WiFi on PN router couldn't even reach the room directly above the BT master socket in one house. That's when I got the TPLINK Archer. Which was a great bit of kit, until I started getting lots of IoT stuff and it kept falling over because it couldn't cope with that many clients.

When I moved here, I had a use case for a wi-fi extender. Tried the cheaper options, they were flaky as hell, so I figured it was time to stop messing about with bodges, and bought the 7590 with a Fritz!Repeater.

At one point I had 89 live client devices across three SSIDs, plus wired endpoints (after cat5ing the whole house) and from that day to this the only router / repeater combo I've tried that can cope with that is still the 7590.

And if I go to FTTP, I can use the 7590. It'll cope with 960mbits/sec of throughput if I tell it to use the ISP's router as the WAN modem. So I won't have to reconfigure the whole house if I stick with the Fritz!Box kit.

[#7]

And if I go to FTTP, I can use the 7590. It'll cope with 960mbits/sec of throughput if I tell it to use the ISP's router as the WAN modem

you don't need a modem on FTTP , the ONT. is effectively the modem. You just connect your WAN port to the ONT with an ethernet cable