Configuration change due to new (secondary) internet connection

Mikrotik
Check Interfaces
Configure IP > Routes
Re-configure default (0.0.0.0) route
Add static route(s)
Configute IP > Firewall > NAT
Enable NAT only for outgoing interface connected directly to Internet
Re-configure Queues
Adjust download speed limit

New ISP's modem
Advanced Setup > Routing
Add static route(s) to internal network (LAN)