I think it's important to note the ":export" is more a script to create a config, than the actual config. And, in scripting, you cannot "add" something if it already exists. So if you have some default config, then do an :import config-from-other-router.rsc, it will conflict with the default.
Assuming you have some users setup, and an export from another router. You can run the import using:
/system/reset-configuration no-default=yes file=config-from-other-router.rsc keep-users=yes
Or if you want to cut-and-paste. Use winbox with the MAC address to connect, then use this (i.e. above without a file):
/system/reset-configuration no-default=yes keep-users=yes,
it will reboot, and reconnect, then you
can cut-and-paste.
But if you have an empty config, likely better
copy your config to the root of Files, and run the import instead of cut-and-paste:
:import verbose=yes config-from-other-router.rsc
The "verbose=yes" option will show you where it's failed if it didn't work (or just show you what it's doing if working).
Main thing is the config needs to start empty, which is what the "no-default=yes" does, an :import should work.
FWIW...Since the config is actually a script. You can modify it to use variables. Basically if you add some:
:global myvar "WAN name"
then you can use a "$myvar" in the export (and now modified) config.
Variables gets more complicated, so more an FYI. But variables allow you to "tweak" the config before using /system/reset-configuration run it if this going to be a recurring thing. And the :import verbose=yes is useful for troubleshooting scripting problems when using variables (or other control structures allowed in the "config")