Can't get started - no Internet

While I’m not a network engineer nor system administrator I am an IT professional and thought I was fairly experienced in setting up a home router situation. I’ve successfully configured many off the shelf routers and switched them from routing to AP & back, etc. I’ve even negotiated and successfully set up an Ubiquiti Edge router. Unfortunately, my recent purchase of a MikroTik router has me completely stumped. I would appreciate some help.
I can’t seem to get the MikroTik router (Mikrotik hEX RB750Gr3, running 6.47.1 RouterOS) to establish itself with my cable modem. I’m a Xfinity internet only customer with Gigabit (DHCP) service through an xFi modem set in bridge mode. BTW, before someone comments, the reason I went with the xFi is that, even in bridge mode, I am not charged a rental, nor do I have any data caps. I currently have an AmpliFi HD mesh router attached to the xFi which then runs to a 16-port switch into various devices. I also have many devices connected via Wi-Fi through the AmpliFi. I want to insert the MikroTik into my network to get better performance for my ethernet devices. I’m not getting full speed due to limitation of the AmpliFi. I know the speed is there because if I take the xFi off bridge and connected direct I do get the speed, but Xfinity has limited almost all configuration options like specifying DNS servers, do any port forwarding, or turn off DHCP and use my own, etc., so I thought putting a full Gigabit router in place would give me the speed while maintain my mesh Wi-Fi coverage elsewhere.
Here is what I’ve tried:

  1. Used the default configuration, unplugged my current router from the cable modem and plugged in the MikroTik
  2. Tried many of the suggested options in various YouTube videos that have you remove the default and grow your own
  3. Tried to fake the system by specifying my currently supplied dynamic address as static
  4. Changed the MikroTik’ s MAC to match the AmpliFi
    Nothing has worked. I do not get any contact from the xFi. It will not grab an address from Xfinity/Comcast. I have viewed the logs, which show nothing other than Ethernet Port 1 is active post any reboot.
    Just to make sure the MikroTik hardware is working, I’ve plugged it downstream from the AmpliFi and attempted to set up a separate LAN. It worked fine. It got an IP from AmpliFi and it gave out new addresses in the assigned MikroTik subnet to the laptop I had plugged into Port 2.
    Sorry for the long post, but I wanted to give as much background as possible. Any help would be greatly appreciated. Or, am I just trying to do something that isn’t supported?
    Thank you very much in advance.