When a user tries to access OWA on a mailbox server that does not host
his/her mailbox, it is redirected to the mailbox server that does host the
mailbox. This is done using NetBIOS name of the server, which cannot be
resolved from outside. Furthermore, were the name even resolvable, typically
the firewall isn't configured to allow access to the other server.

Options:
1) Install a FE server or
2) Allow access to the second mailbox server from outside:
- Configure your firewall to allow access (HTTP/S) to that server from
outside. (Usually involves a NAT rule that maps the internal IP to an
external IP, and an access rule that opens the appropriate port(s) - in this
case https=443 assuming you're using SSL - for inbound traffic to that
server).
- If the external dns zone for your domain doesn't already have an A record
for that second mailbox server, create one so it maps to the external/NATted
IP address of the second server.
- Instruct users with mailboxes on second server to access it using its
fqdn/A record