Most dual-SIM phones only support 2G for one of the SIMs. Some phones (all dual-SIM Windows Phones, and some Android phones) let either SIM slot be the primary SIM, and it's a software toggle. It seems that's the way the OP's works, from what was said. So the simple answer to the OP is, yes, only a SIM on the Rogers network can be the secondary (non-data) SIM on the current device.
Options using two devices, accepting that the primary SIM must always be the one on Bellus, or upgrading to a newer and higher-end dual-SIM phone. The reason dual-SIM phones didn't support the second SIM being at least 3G until now wasn't because they were cheaping out, it's because most phones use off-the-shelf system-on-chip (SoC( designs with integrated modems, and those modems didn't support it. My HP Elite X3 supports the secondary SIM connecting to HSPA because the modem in the Snapdragon 820 SoC supports it. Even at that, the feature wasn't enabled at first: users of that phone had to wait for a firmware update that enabled it.
Another very important consideration is that while Rogers continues to support 2G GSM for now, and has not announced an official end-date for 2G support, that 2G coverage hasn't increased in a very long time. And 2G equipment is beginning to fail as equipment eventually always does, and that means the 2G footprint is actually shrinking (though Rogers appears to be moving equipment around still, to make sure key areas are covered). Nonetheless, using Rogers as the secondary SIM will be a worse and worse experience over time. U.S. roaming won't work with 2G, unless you force your device to roam on T-Mobile. Both Public Mobile and Rogers support roaming on both AT&T and T-Mobile, and in my experience it defaults to AT&T most of the time.
So, carrying around two devices, or acquiring a newer and probably very expensive dual-SIM handset, are really the only two good options on the table.
