The configured IP address can reach the internet?
You can get support to enable BGP by logging a ticket, but I'm not sure how fully featured it is. I believe it is better than the OSPF listen only routing.
What was the config at the 4xxx?
Did you check the next hop if it has the correct arp entry, meraki mac+ configured ip .
Does the provider block/filter any traffic related to this test? https://documentation.meraki.com/MX/Firewall_and_Traffic_Shaping/Connection_Monitoring_for_WAN_Failo...
The MX84 can not do BGP for this case.