Have you tried changing the other ports (2-5) to “master-port=ether1”? Maybe ether1 doesn’t like to be a slave port. All the ports except ether1 show “none” for the master-port if you remove them from the switch.
ADD: Since the RB750 has no serial port, you will need to get logged in on ether1 to do this. If you log in to any other port, it will disconnect when you remove the port from the switch, or in ether2’s case, when you add ether2 to the new switch (set ether2 master-port=ether1).
To clarify:
RB750 ports 2-5 can be switched together;
RB750G ports 1-5 can be switched together;
RB450G by default ports 1-5 can be switched. Supports “switch-all-ports=yes/no” that lets you remove switching ability from ether1, but increases ether1 troughtput to other ports in bridged, and routed mode.
Ether1 on the RB750 is to be used as a WAN type port (gateway) to a ADSL/VSAT modem.
By default it is named as “ether1-gateway” on new RB750’s. It will must have an IP address that allows it to communicate with the attached modem port. (Subnet/Network and Broadcast must match)