Basically a non modified modem is a legit modem from a well known brand that works as expected, allows you to use a sim card to get online

now, telcos here that offer unlimited plans usually block those sim cards from being used in modems because the usage when shared is going to be much higher than you using it in youir phone

modified ones trick your sim/teclo into thinking its being used on your phone so that the unlimited plan can be shared
with that said they do have methods to detect it and high data usage will probaby raise suspicions

for your use case of roaming... some roaming providers will give you a modem and you give it back to them at the end
could consider that instead of buying your own