Not exactly sure where to post this but this forums seems appropriate.

I have a U.S. SIM (a MVNO using T-Mobile roaming arrangements) that I use for data access from time to time. T-Mobile have two roaming partners in NZ, Spark (3G/4G) and Vodafone (3G only roaming). Since I use it mainly for data access, I usually force it to stay on Spark's 4G network, as 3G roaming is usually unbearably slow.

However, the connection is unreliable at best.

Problem starts with registering with the Spark network. It almost never works first time, and sometimes I'd have to leave it be for a couple of days before the SIM could register on the Spark network. I've noticed, however, that:

    1. If I disable 4G, the registration process tends to be more consistent
    2. When switching from 3G to 4G, or when registering on 4G directly, I must be an area with FDD-LTE B28 coverage. It appears that the Spark network only permits the SIM to use the B28 network. I could reliable reproduce this with Mobile Signal Guru -- if I disable B28, it'd be stuck on No Service, if I disable all other bands and leave only B28 enabled, the registration would go through fine
    3. Once attached to the 4G network, switching bands doesn't seem cause any issues -- however, a move between cell sites could leave me with No Service again

Even if I stay stationary, there's a chance that service might drop after a few minutes or hours. It seems completely random to me.

I should mention that Spark has stellar coverage in my area, I also have a SIM from CMHK, which stays on the Spark 4G network with no issue at all.

I've tried reaching out to the carrier (blamed T-Mobile), T-Mobile (blamed Spark), and Spark online chat, but have so far yielded nothing. Perhaps someone from Spark could shed some light on this issue?

Thanks anyways