New Router = Can't Find RN424 drives

First log into the admin web ui of the NAS and navigate to the network page.  Click on the settings wheel next to the graphic.  (There will be more than one graphic, the one you want is the one that is not grayed out).

There will be a pop-up with three tabs at the top.  Pick the one that says IPv4 (which is in the center).  If the configure field doesn't say "Using DHCP" then  set it to that.  Then power down the NAS using the power control in the upper right of the web page..

Then you can switch routers.  After the switch, power up the NAS.  After it boots, you should be able to see its new IPv4 address on the LCD panel.  Make a note of that address.  It will most likely be different than the IP address you were using with the old router.

You can go into the tmobile gateway settings, and reserve that address for the NAS, so it will always get that IP address every time the system restarts.  (Best to download the gateway manual and look for that setting).

Then try opening Windows File Explorer and enter \\NAS-IP-ADDRESS (using the actual address you noted above, instead of the NAS-IP-ADDRESS placeholder).  You will need likely to enter your NAS user credentials again.   See if that opens up your list of shares, and confirm that you can access them.

Since the IP address will be different, you will need to re-do the mapped drives on the PC(s).