Unstable Internet connection

My company currently is using ZyXEL router modem, always during early
in the morning (1or 2 people in the office) and after working hour (4 -
5 people in the office), the network is fine. But during peak hour like
in the afternoon (30 users in the office) ... the Internet will always
down.

The router have the username and password configured in it to connect
to the broadband Internet connection provided by our ISP. The DHCP is
enabled, all user is getting IP from the router DHCP.

Anyone experience this before?
Does it related to the modem or the ISP?

Question:
1. If concurrenly 30 users is using the connection, will is be possible
the Internet connection is down caused by the incapability of the
router modem to support that many users? If this is that case, any good
recommendation?
2. If (1) is not the case, what would be the possible reason? The ISP
problem?

By the way, does anyone know is SMC wireless router modem perform well?



Peter CCH