How do I get a SIM card from AT&T? (Max BR1 Pro LTE) The rep I chatted with claims they don’t support this router.
I believe you just have to ask them for a full size SIM card. They don’t need to know what router you are using.
Sometimes the full-size cards are unavailable, in which case there are plenty of adapters avilable (we use isyfix adapters, fwiw).
Some carriers do not allow unapproved equipment onto their networks, e.g., either by not activating the SIM card when initially installed in the unapproved unit (solution - activate it in another unit first) or by directly blocking the unit when it tries to connect (even if the line has been activated).
In your case this should not matter: MAX-BR1-PRO-LTE-US-T is approved by AT&T (https://www.peplink.com/products/max-cellular-router/single-cellular/#br1pro) so it seems the rep you spoke with is ill informed.