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Have you looked at some of the self help tips via the Vodafone-Sure-Signal-troubleshooting.
It may also be worth trying a different power supply with the SureSignal (Version 1 & 2 ), even if it still looks ok regarding lights etc as there have been some reports of replacement power supplies solving the issues as they can degrade.
They are available on eBay for around £10.
From member cineriv "But firstly, have a look around for a suitable unit with a 12 Volt output @ around 1.5Amp capacity that you could 'borrow' to prove the fault.
Suitable units can be found on external hard drives, monitors, printers, routers etc.etc."
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.