I checked the manual and couldn't find the settings for a bridge mode. I would change the device to a modem without wifi and which can be set up in a bridge mode.
Alternatively the best shoot is probably to disable DHCP and firewall and choose a network different from your LAN after the RG450 and put your RG450G in the DMZ.
example:
In the SMC choose LAN to be 10.10.0.1 / 255.255.255.0 disable DHCP and firewall and setup DMZ as 10.10.0.10
Then set up your RB450 ether1 (as the wan side) interface with a fixed IP to connect to the DMZ
it sucks, but may work
/ip address add address=10.0.0./24 interface=Public
/ip dns set primary-dns=10.10.0.1 allow-remote-requests=yes
/ip firewall nat add chain=srcnat action=masquerade out-interface=Ether2
/ip route add gateway=10.0.0.1
Set ether2 as your master lan interface and set ether3-ether5 as slaves
/ ip pool add name=dhcp-pool ranges=192.168.0.50-192.168.0.200
/ ip dhcp-server network add address=192.168.0.0/24 gateway=192.168.0.1
/ ip dhcp-server add interface=ether2 address-pool=dhcp-pool
the firewall could be something like this
/ip firewall filter
add action=drop chain=forward comment="" connection-state=invalid disabled=no
add action=accept chain=forward comment="" connection-state=established disabled=no
add action=accept chain=forward comment="" connection-state=related disabled=no
add action=accept chain=forward comment="" disabled=no dst-port=7345-7346 protocol=tcp
add action=accept chain=forward comment="" disabled=no dst-port=20-21 protocol=tcp
add action=accept chain=forward comment="" connection-state=new disabled=no src-address=192.168.0.0/24
add action=log chain=forward comment="" disabled=no dst-address=192.168.0.0/24 log-prefix=UNWANTED src-address=\
!192.168.0.0/24
add action=drop chain=forward comment="" disabled=no dst-address=192.168.0.0/24 src-address=192.168.0.0/24
add action=accept chain=input comment="" disabled=no dst-port=22 protocol=tcp src-address=192.168.0.0/24
add action=log chain=input comment="" disabled=no dst-port=22 log-prefix="" protocol=tcp
add action=drop chain=input comment="" disabled=no dst-port=22 protocol=tcp
I am probably missing something, but I hope it may offer some help with your set up.
PS. To enter above rules just start winbox and start a "new terminal" window and paste the lines with the rules that you want to enter.