I think no one can pin point the cause of problem for you because we need more information ex. network diagram, how you exchange routing with both ISPs, IP address you using & who own those IP, ..., etc. How about the reachability from your FW to both routers?
From the information you gave us, 1st router & 1st ISP network was working properly because you can go outside by using their IP address on serial line as source IP.
The cause of problem may come from your routing policy setting up by both ISPs. Since you do not run BGP to exchange routing with both ISPs, it is very difficult to identify the problem
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...