3 problems dude:
1) shub1
2) wifi on shub1
3) powerlines
For starters shub1 is all round pants and I wouldn't expect it to handle traffic to seven devices simultaneously with any degree of competency. Secondly the wifi is absolutely diabolical on shub1 and I would honestly just rule out using it full stop. Thirdly, powerline adapters are very flaky. Wireless is variable anyway with regards to interference between yourself and the router/extender but then there is also interference on your electrical circuit which can completely kill your connection between the adapter/extender to the router. The fact that your laptop/pc is saying connection is limited means your connection to the router is dropping.
There is no easy to fix this if you are living in student accommodation and renting. For starters I would do two things.
1) Take a laptop downstairs and directly connect to the shub through one of the ethernet ports on the back with a patch cable and run some speedtests. If this works and your connection remains stable then you know it is the wireless which is letting you down.
2) If that fails I would put the shub in modem mode which will kick everyone else off the network and try the wired test again. The laptop/pc connected to the shub will be the only device with internet access.
In an ideal world the following would solve all your problems: put the shub in modem mode and buy a proper router like
this. It absolutely kicks ass and wouldn't have a heart attack like the shub does when lots of people want to use the internet at the same time. The wireless range would be significantly better and go a long way to improve your current situation. To make things perfect I would either run some patch cables from the router under the carpet to rooms around the house in order to provide direct wired connections or as a minimum one cable upstairs to a second router which can serve as a wireless access point and further extend the range and signal throughout the house.
It is worth thinking about. If you are there for the rest of the academic year it would work out around £20 each for you and your pals for either option and from a cost/benefit point of view I think it is money well spent and if you can prove it works perfectly (which it will) then you might be able to convince your landlord to reimburse you at the end of the year. Tbh though, for the amount of time I spend on the internet and playing games I would be happy to cough up the dosh myself.