No landline or broadband for 14 days

[#1]

@jab1  This is certainly an odd situation, I am suspicious when the OP says "when I try to log into my WiFi I receive a message"  If there is no Plusnet broadband connection and very unlikely to be any connection to TalkTalk either so where does this message show up and where is it coming from?  Has the OP inadvertently connected to a nearby TalkTalk router?

As you say I don't believe we have the full story.

[#2]

Briefly looking at the guides on setting up TT routers, there doesn't appear to be any account details required - just plug it in and go. So it is just possible that a Plusnet (or other) router would gain a connection - at least to a walled garden - without any configuration.

[#3]

TT use DHCP to make a WAN connection and get an IP. Plusnet routers are fixed to using PPPoE and so would not connect. A non Plusnet router previously configured for Plusnet MAY establish a connection after a factory reset, if its default WAN protocol is DHCP.

[#4]

This narrative is somewhat confusing.  If attempting to place a telephone call results in a TT calls barred message, then there’s certainly the possibility of a crossed or wrongly connected d-side circuit.  Fixing that is very likely to require a trace and test from the premises … and a final end to end connection test.

Getting a TT service connection barred on WiFi suggests an entirely different.  Whilst router might establish a DSL sync session on a cross connected network … to get to the point of receiving an account blocked message requires the establishment of a PPP session having passed logon authentication.  Such seems wholly unlikely.

However what is very credible if (unsecured) Powerline adapters are present in the property and a neighbour’s is the possibility that the connection is being routed through the neighbour’s network.

[#5]

---

@Townman wrote:

However what is very credible if (unsecured) Powerline adapters are present in the property and a neighbour’s is the possibility that the connection is being routed through the neighbour’s network.

---

I like that as a possible explanation!  Although it would require two separate issues - one phone and one WiFi - although it is still as plausible as any other diagnosis we've managed to come up with.

It would require a neighbour in the same phase as the OP, which may not be the immediate neighbour.