I'd try bypassing the router and run one device hard wired and straight from the modem. You say it takes 3 to 5 days. If you can run for a full week without this problem, you've narrowed your search considerably. Once the "let's try this" approach fails over and over, it's time to take a breath and troubleshoot using logic.
Since June 2015 I have been experiencing at least once a week my internet connection dropping. I purchased a Linksys wifi router at that time so I thought that the router was the issue. I can't tell you how many people with Linksys I have chewed out because the troubleshoot done and 3 replacement routers didn't fix anything. We have changed the channels so many times and that didn't work. The tech with our ISP is at my house 1-2 times a week. We have replaced all wires, modems, amps, and just recently bought a different brand wifi router. At even 3 days went by with the new router the connection drops again. I contacted the company at told them the issues. The wires to the modum are not split. Once I hardware my laptop to the modem and do what needs to be done with the wifi company to get internet connection again it works fine for 3-5 days and it happens again. We got the new router 3 weeks ago and I already have had to call regarding this issue 4 times. Nothing is working and apparently my ISP that provides the modum or the company that makes the router have no clue what's causing this or have a clue what to do! 8 months, every week, 2-5 hours a day to my isp and the wifi company is getting extremely old and I don't have that kind of time to continue this. Please someone help me find out the cause of constant dropped connection issues before I lose my mind!!! Lol