Can't Access My Asus RT-AC68u Routers

I have one main router which is a fibre-optic Huawei setup on 192.168.2.1. Then I have two Asus routers, one acting as a wireless router and one as a node (both RT-AC68Us). The main Asus router has a WAN address of 192.168.2.5 - issued by the Huawei and set up with a LAN address of 192.168.1.1 The node is on 192.168.1.217. I set this arrangement up a couple of years ago and now want to incorporate a VPN. The problem is I cannot access either Asus router. I have tried opening 192.168.1.1 - that just times out and http://router.asus.com comes up as "Can't be reached" I can't remember anymore how I set it up the first time. I have the Asus Router mobile app but seems to only allow a limited number of things and I can't find a way to use it to get into the main setup area for the router. Can anyone put me straight? Many thanks. Richard