I have a data SIM with 80 gig data and 365 day expiry. I had it in a Telstra unlocked pocket wifi and it worked well until this modem died. I replaced this modem with an Optus Huawei E5577 pocket modem 2 months ago and it worked well until last week when the modem now says "No Service". I had the SIM replaced and eventually the girl in Optus shop got it to work but now it it is saying again "No Service" Is this a common problem with these modems, I'm thinking it is a modem issue. Would a prepaid SIM work in a Huawei Wifi Modem B818? These are only usually sold with a plan SIM.
If you purchased the E5577 only recently and it's stopped working you shouldn't need to go and purchase another one.
If you got yours hands on a B818, no issue to use with a Prepaid SIM. You may just need to change the APN to preconnect if using Optus.