I have a microtik rb951ui-2hnd and I have configured it as CPE. The internet input is via WIFI, connecting to the SSID. and the LAN uses this connection for nat. I want to know how to configure the Internet input with several WIFI connections configured and stored. In case a WIFI connection fails, it connects to a second Wi-Fi connection automatically. This is possible?
There are connect lists, see:
http://forum.mikrotik.com/t/auto-connect-to-different-ssids/141473/1
http://forum.mikrotik.com/t/travel-router/144678/1
The issue is that there are two possible cases of “no internet”:
- the Wi-FI AP fails
- the connection to internet fails beyond the Wi-FI AP
If #1 the particular SSID your CPE is currently connected to will disappear and there will be an attempt to connect to another SSID, next on the list.
If #2 the CPE will remain connected to the current SSID as it is working just fine only you will have no internet, this is the same case as - in wired networks - the connection is lost after the local gateway.
So for #2 you need something like recursive failover routing or a netwatch script, at first sight (but I may well be wrong) the recursive approach seems to me more complex (if possible at all, maybe if you control all the various SSID’s and can use static IP’s, with different networks on each SSID then it can be done, but cannot really say), so I would personally make an attempt with a netwatch script, BUT - just thinking aloud - there will be an issue in detecting when the internet connection has come back to the “main” SSID.
Your device has only one 2.4 GHz radio, so to “probe” the “main” SSID you will need - I believe - to disconnect temporarily from the current SSID (and thus lose connection), so you cannot do that very often. 
Let’s wait for some of the more experienced members to come up with better ideas, in the meantime you can check/study/test the connect list approach.