(I know that I should have just used a mesh network)
I have a router from my ISP, and it is placed at one end of my house.
My Chromecast, printer, and other IoT devices are connected to it. I recently connected another router via Powerline to the other end to get quicker speeds, and to get Ethernet for my PC. However, I can't print off of the second router's network, and I can't use the Chromecast virtual remote. The router shows up as a single device on my ISP router, and none of my other devices connected to it show up.
Do I need to put the router in bridge mode, or do I just need to mess around with the configuration of it?
Will the router support 5GHz still? (My ISP router is only 2.4GHz) \
Will this disable 5GHz wifi and the ethernet ports on my Linksys router, and will the Linksys router's control panel be unreachable once in bridge mode?
It depends on the specifics of the router and configuration, but it doesn't have to. It won't be your "gateway" anymore but it might be accessible on it's own ip. Same with 5ghz no reason it has to be lost
Your network is behind a second NAT. You need to put your router in bridge mode if you want to have the dhcp and firewall disabled.
Honestly the best answer is to get a dump network switch and plug in all of the devices to it. From there you should configure the WiFi router as a dump AP.
Personally what I did was install openwrt on two devices and then setup roaming with 802.11r