Apologies, If I didnt make this clear
The TP-Link and Huwei are not connected to each other in anyway
Two entirelly seperate routers, one feeding the Wi-Fi via Mobile PN the other via FTC
I understood that. It seems nobody else has done.
They also query what you mean by the "PN Server". I assumed you meant the Plusnet core gateways/routers and servers. Others seem to think you mean your Plusnet router at home.
The confusion seems to be whether you mean your FTTC router loses sync, or just the wifi to it fails.
My original point was that the gateways at the Plusnet end cannot accept two different routers simultaneously connecting to your
Plusnet account's landline service with them. It has to be one or the other. Your EE Mobile Broadband, though provided for sales and accounting purposes by Plusnet, will (as delivered) be connecting to EE's mobile data network. Not Plusnet's landline network.
As I suggested earlier, your declared aim seems to be to ditch the FTTC if the Mobile Broadband service proves adequate. If I'm correct, your only valid way to test is to transfer your wife's internet connection to it along with yours. Giving it the full "normal" load that the FTTC used to have.
While you still have the FTTC there to swap back to.
Is the TP-Link supplied by your wife's employer?
Connections: OnePlus 8 Pro on Three 4+ (LTE)/5G and at home Three Mobile, with (Three)ZTE MF286D router giving about 113/20Mbps.
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“I have hardly ever known a mathematician who was capable of reasoning.” (Plato)