You may have misunderstood. Sounds to me that TVeblen
was simply implying that you would probably not have a problem with splitting the signal (in other words, the signal will be cut in half, but the connected devices will likely still be able to use the half-strength signal).
As far as the suggested test, I think TVeblen
may have misunderstood the fact that your router does not have ethernet ports and is only wireless. What you could do, is connect (via ethernet) your CPU directly to the modem to test (but before you do, make sure your firewall, AV, and MS updates are in good shape). That would help you to determine if the router could've been the cause of your trouble.
So when that spltter arrives you're going to split the modem output to your router and CPU? Meaning your CPU will essentially connect directly to the modem? That would mean you lose the security protection afforded by the router. If it was me, I would insist on placing a router between the modem and CPU.