I use a VPS service that, along with the virtual machine, also provides me with a series (not contiguous) of static public IP addresses that I can associate with the interfaces of the virtual server. I also have a Mikrotik router (router A) connected to both optic fiber and another 4G Mikrotik router (router B) with a SIM card that I use as a backup in case the fiber connection is interrupted.
I would like to configure "router B" to provide a chosen public IP address from those provided by my VPS service to the WAN interface of my "router A," so that I can always have a public IP directly on router A even when the fiber connection is interrupted.
I would like the connection to not simply be a NAT via a VPN connection to the VPS, but something that may make use of PPPoE or similar protocols to assign the public IP directly to the WAN of my router.
Is there a way to achieve this? What technologies and protocols should I use?

Maybe the simplest method would be to directly assign a /32 address of your choosing from the public ones to the Router A facing interface with Proxy ARP