Sorry for digging but i want to provide a solution for this problem.
(and sorry for my English)
From my experience the problem with accessing Linksys routers interface is caused by outdated firmware.
At some point Linksys removed captcha image from the router page where you can start creating Your Linksys SmartWiFi Account and the story began...
So the firmware changed significantly...
I found that even EA6XXX that was shipped in 2013 (i dont remember FW version) suffered Bad Gateway problem.
Autoupdate feature was unable to download and update new, lets call it "new release" FW version and was saying that you have the latest FW.
The solution is pretty simple
: (this way i fixed like 10 Bad Gateway errors permanently on Linksys routers)
1. Disconnect all cables from the router except the power cable, have the router turned on and stable.
2. Reset the router by holding reset button (the red one, not the blue one, of course ;P) for 20 seconds.
3. Wait for the router to stabilize.
4. Connect your computer by cable to one of the router Ethernet ports (one with number 1,2,3 or 4, NOT Internet port) (WiFi should work also but i prefer cable connection ;P)
5. Open your web browser and access the router page (enter 192.168.1.1 into address bar and press Enter ;P).
Now... if you will get Bad Gateway error again just try with different browser, actually in Windows 10, Microsoft Edge is the best i find and always reach the first setup page.
5. On te initial setup page go for Manual Configuration (2nd option), tick ToS and press Next.
6. Do not choose to login with SmarftWiFi Account but click on "For local access click here" under language choice drop-down menu.
7. Now you should be asked for Router Password (pass only, no username), the pass is "admin".
8. Now, (unfortunately for some ppl) you need internet connection to download the latest Firmware for Your router.
9. Go to support.linksys.com and enter Model Number of your router (it can be found on the sticker that is usually under the router, Model No, also look for version number if there is, sth like v1 or v1.1 after the Model Number !)
10. There should be only the Model to choose on the list so click it.
11. Now you should be on your router Linksys webpage.
One like this :
http://www.linksys.com/us/support-product?pid=01t80000003KdHUAA0
12. Now you have to reach Downloads page, do it by scrolling down the page or use 3rd widget to the right. Click on "Download Software".
13. Choose your router version. (remember it was on the label under the router
).
14. Download the latest Firmware.
15. Connect your computer to Linksys router.
16. Login into the Linksys router.
17. Go to Connectivity, update the Firmware Manually by choosing the downloaded firmware file and starting the upgrade.
phew..
Now you should be ok and Bad Gateway error issue should be fixed.
Now you can create SmartWiFi Account as well.
Hope it helps, let me know if not.
GG&GL