Update: The owner of the Realm has turned off the one resource pack that was enabled, although this has not caused an issue before, nor is it an issue for other players or my account on a different computer (laptop). This issue remains on my main PC.
My PC is wirelessly connected to the router, whereas my laptop is wirelessly connected to the Mesh. Today I tried connecting my PC via the Mesh and managed to get on to MC Realm!! Pure fluke that I tried it tbh.
The router is only 2 months old and is still perfectly fine when streaming video, and playing other games (including via Xbox Cloud Gaming), additionally, speed tests run from my PC wirelessly to the router show a perfectly clean connection, so I did not think this was the issue. I'm actually surprised MC Windows is so susceptible to such small internet fluctuations, which is evidentially the case. Although this now makes sense with others reporting this fault just seems to repair itself.
You can now close this bug report.