I got a MyRepublic SIM a few weeks ago, with unlimited data for $25 at 2Mbps, to use with my Asus 4G-AX56 router, but when I popped it in it refused to connect. Switching it out to a mobile phone worked fine including hotspotting. 

I contacted MyRepublic's live chat support and they confirmed that 4G routers are not supported but encouraged me to use a phone hotspot instead. They didn't see this as a problem. 

I'm guessing they're using the TTL to confirm that it's being used by the router not the phone. If this is indeed the case, is there any way to change the TTL in my router to get around this?

I can't imagine this violates any ToS, since the use of (or lack of support thereof) 4G routers is not mentioned anywhere in MyRepublic's ToS. And hotspotting is a feature of this plan, which is basically what a 4G router is doing.

Yes I could use a different provider, but the next best unlimited data plam is Skinny's $40 p/month. I don't need super fast speeds just the peace of mind of having unlimited data