What J0hn said.
I use the ZyXEL VMG 1312-B10
A, not -D which is inferior, according to the consensus. I use this device in modem-only mode as you are proposing to do.
The 8- series ZyXEL models might be easier to get hold of than the VMG 1312-B10A and possibly cheaper too because the VMG1312-B10A is not being made any more. I’m not exactly sure, but I think summer 2017 might have been when ZyXEL ceased offering that model. Those alternative top end models might be less hassle.
VMG1312-B10A devices can be found on eBay but sometimes the prices are silly, sometimes not. Very occasionally, new ones could still be seen on eBay, within recent months.
I very much recommend the
custom firmware that our member Johnson has built for all the various ZyXEL models. Amongst other enhancements, this firmware gives you full 1508 byte PPP MTU when in modem-only 'bridge' mode, provided that your router and service provider can handle 1508 byte long PPP packets. Without it you have to live with 1492 byte MTU.