hEX has 5 ethernet ports, marked on the outside as "Internet/2/3/4/5".
Default config follows this - ether1 is configured as separate interface that is meant for WAN/Internet access, ether2-5 are a bridge for LAN devices (so each LAN device can talk to each other).
Enabling the "bridge all ports" means "all ports are LAN", in basic terms, so port1 is no longer special.
You would use this if you want to use the hEX as a dumb switch (because you already have the hardware) or use something slightly more complex/advanced for WAN/internet access (PPPoE, tunneling, VPN, LTE modem in USB port, etc).
That said, start with upgrading RouterOS, because master-port is an outdated concept so you probably are running old version.