I have RB750 and removed default script.
I have static IP from ISP 206.75.200.154 and Gateway of ISP 206.75.200.1

I added next 3 lines to configuration:
/ip address add address=206.75.200.154/24 network=206.75.200.0 interface=ether1
/ip route add dst-address=0.0.0.0/0 gateway=206.75.200.1 disabled=no
/ip dns set servers=8.8.8.8,8.8.4.4 allow-remote-requests=yes

Now I am expecting to have Internet in the router. So I click "New Terminal" in winbox and start pinging IP addresses.

I only get timeouts. What am I missing?

first of all - are you able to ping your default gateway 206.75.200.1?

just in case the router is trying to use a private ip address that you may have on the router, make sure you specify the src-address in the ping command
ping 206.75.200.1 src-address=206.75.200.154

see if you have any other default gateways configured on your router, and/or a more specific route to the ip address you are trying to ping

if you are able to ping your default gw - do a traceroute to say 8.8.8.8 (remembering to specify src-address), and see where the trace stops?

If you don't have any luck, please post results from following commands
ip route print
ip address print
ip firewall nat print

check your subnet mask,did the ISP gave you /24?

When I added the DNS like:
/ip dns
set allow-remote-requests=yes cache-max-ttl=1w cache-size=2048KiB \
max-udp-packet-size=512 servers=8.8.8.8,8.8.4.4

It all worked fine.

I guess when adding static IP's, you'll need to define the DNS server.
Thx for all your help.