I’m not sure I understand the last part. Both 192.168.1.1 and 192.168.8.1 should be pingable from RB. If not, on Ubi router it would be caused by your config. And for LTE it’s unlikely that manufacturer would block ping from LAN.
For failover, there are different methods. You can add route via LTE also to main routing table (without routing mark) with higher metric, add check-gateway option (ping or arp) for all four routes and you have the simplest config. Or you can go for Advanced Routing Failover without Scripting or anything else you like.
And you can use this as optimized srcnat rule, because you can keep original source addresses for connections using WAN1:
/ip firewall nat
add action=masquerade chain=srcnat connection-mark=wan2conn
You could also get rid of it completely, if you merged both subnets. Since there no separation anyway, LTE could be e.g. 192.168.1.2/24 (if it’s not already used by something else).