"one cable goes from the modem to a splitter which goes to the wifi router on the top floor and works perfectly."
While the details here are missing, there are few cases where this would be right. A modem goes to the router. Not more than one device.
If you omitted a detail here and this is an integrated modem+router then your setup is again unsupported. Let's get the details down pat first.
Ok so I've got kind of a weird problem. Its been going on for a long time so I'll start from the beginning :).
I've got a cable running from the basement up to my living room where the wifi router is plugged in. The router works really well and the internet is fast. Sometimes however the router will loose it's connection to the internet for no reason. I'll be connected to the router but not to the internet. To fix this I have to reset the router. Resetting the modem doesn't do anything. sometimes it works perfectly all day but then the next day it might disconnect 20 times.
I tried fixing it by placing a new router but the new one does the same thing.
I tried connecting it straight to the modem (instead of the cable going up to the living room) but this didn't do anything neither.
The strangest thing is that i've got another wifi router on the top floor that works perfectly fine (the one I replaced for the new one downstairs and didn't work downstairs).
So one cable goes from the modem to a splitter which goes to the wifi router on the top floor and works perfectly.
But then there is a cable that goes from the splitter to my living room and that one doesn't work.
Different cable from that splitter to my router doesn't work and skipping the splitter and plugging the router straight into the modem doesn't work.
I honnestly don't know what else to try.
Sorry for the long post
Would love to get some help on this!