"Internet Partial Service (DS only)" and low Downstream "PLC Power (dBmV)"

Before Christmas, we started getting random devices dropping off the Wifi. An engineer visited and swapped out the Hub 3 for a Hub 5.  He did comment that it might be caused by the number of devices we have for the speed we pay for (approx. 18 devices on the M250 broadband package).

The Hub 5 has been working perfectly until the last few days whereby all of the internet is dropping, on all devices, included my ethernet wired desktop PC (before Xmas, it was only wireless devices that were affected).  We've upgraded to the M350 package and waited for the speed increase to kick in, but I still suspect something isn't right with our connection.

I notice the Hub 5 status page is showing "Internet (Partial Service (DS only))", which we also saw when we had issues with the Hub 3 before Xmas.  However, when I've dug into the router stats in more detail and from googling on these forums, I'm seeing this in the 3.1 Downstream channels section (I can't post images for some reason)

Channel ID 159

Locked Status Locked

RxMER Data (dB) 0

PLC Power (dBmV) -25.3

Corrected errors (Active Profile) 1984177

Uncorrectable errors (Active Profile) 1150132469

...which from what I've read, isn't good (power too low) and lots of errors.  I've rebooted the router a couple of times, but the errors still start coming back.

I suspect an issue with the cable coming into the property as searching for any faults in the area returns nothing.

Do we need an engineer to come to the property to resolve this issue?

Even further back, we had to get Virgin to come and fix an issue whereby we lost channel 5.  Apparently, water had got into the cable by the front door.  The engineer did say that eventually, they may need to replace the cable coming into the property as it couldn't be shortened any further, so wondering whether this is going to be what's required.

Thanks in advance for any help.