Hello,
You can control to which operators the SIM card will able to connect and how via the Network → Mobile → Network Operators page (more info here).
If you have a multi-operator SIM card, by default it will connect to the operator with the best connectivity.
By "non-standard APNs" do you mean the ones that operators give out under special contracts? If so, you can still enter them via the Network → Mobile → General Settings page, APN field. (here)
If you want use more than one APN at a time, I'm afraid that is currently not possible with RUT routers. If you want to use different APNs on different occasions, you can change the APN with an SMS message (info here) or create a different configuration profiles that use different APNs and switch between these profiles whenever there is a need (info here).