The problem is likely AT&T. The L5’s US modem isn’t one of AT&T’s (few) whitelisted devices. Even though you’re using an MVNO, the MVNO has to comply with AT&T’s device requirements. I’m guessing you’re using a SIM that was already activated in a separate, authorized device, right? Otherwise, AT&T (or MVNO) would not have activated it in the L5.
One problem, too, is that even if AT&T doesn’t eventually block your SIM, you probably won’t be able to get VoLTE working on AT&T’s network, since the modem isn’t approved. Additionally, AT&T has no 2G or 3G network for calls to fall back to in the absence of VoLTE (voice over 4G LTE), so this will render calls inoperative.
T-mobile and related MVNOs, however, do approve the modem’s IMEI for activation and use on their network, and it does get VoLTE, and even has 2G fallback. VoLTE can be enabled on the L5 via the BM818 Tool app.