Internet only works between certain hours?

My internet connection has only been working properly between roughly 10:09am and 4:01pm every day, and this is now day 5 of the issue. Outside of those hours the service is basically unusable due to the upload speed dropping.

I have run multiple speed tests and the pattern is exactly the same every day. Before 10:09am and after 4:01pm, my upload speed sits at around 0.20 Mbps, but between those hours it jumps to around 40 Mbps.

I have made countless calls to Virgin Media and am getting nowhere. An engineer is due to visit today between 12pm and 4pm, but I am not hopeful as this clearly is not an isolated fault. I live in a block of flats and multiple neighbours in my block, as well as the block next door, are all affected by the same problem.

One neighbour already had an engineer visit yesterday and was given a replacement router, which only worked for about 10 minutes before the issue returned. I am using a Hub 5 and have only been with Virgin Media for two months, so this is incredibly frustrating.

Has anyone experienced anything like this before or got any ideas what could be causing such a specific, time-based upload issue? Any advice would be appreciated. Thanks!