Q:

One ip on two-WAN router with load balancing (Asus RX3042H)

I want it to balance traffic for the network, but one computer needs to walk on the Inernet from one ip (now it appears sometimes as ip1 and sometimes as ip2). Still it needs to switch to another channel, if default is broken. How can I achieve this?

A:

If you want the same IP even when failing over to another ISP you have to use BGP and get IP space assigned to you (ARIN in north America, RIPE for Europe, APNIC asia-pac...), this is not cheap.
Then there's ~10k worth of routers, and fixed ethernet/fibre links (as very few carriers will do BGP over *DSL, and none over cable)
Most of the simple "fail-over" routers can be set to a fail-over mode instead of load-balancing, the better ones even have rules so some sorts of traffic always prefer one interface over another.
Your router can be set to this mode, see section 5.6.1 of the manual:
http://files.wl500g.info/asus/rx3042h/manual/RX%203042H%20manual_EN.pdf

