Can this work? Bridge / Wan, several routers

You could string them that way but you will get a massive mess with all the ip addresses and subnets. It would be better to use the LAN ports and use the routers as AP if they have wireless. If they have no wireless then a simple switch will be simpler.....the lan ports internally are really a switch in a router. As long as the cables between the devices are 100meters or less it will be fine. There likely is some limit to how many devices you can daisy chain but it is some huge number.

Combining connections is not that simple if they have different IP addresses. There are method to bond say ethernet ports together but not sure if that applies in your case. If you control both ends of the connection there may be other options to combine them

I presume each of these hotel rooms has a phone? The way most hotels deal with this problem is to buy a DSLAM (or several DSLAMs) usually running VDSL. Put the DSLAMs in your phone room, connected to the Internet. The DSLAM ties in to the phone lines going to each room. In each room you put a VDSL modem. Over short distances like this, VDSL is easily capable of 20+ Mbps. The VDSL modem converts the signal over the phone line back into Ethernet that your customers can plug into. Basically, you're piggybacking ethernet over the existing phone lines going to each room, so you don't have to install new network cable. It's just like regular DSL (although VDSL instead of ADSL), only using your hotel's internal phone lines instead of the phone company's phone lines.

https://en.wikipedia.org/wiki/VDSL

The downside is cost. The VDSL modems will run you $100-$200 apiece. The DSLAMs will vary depending on how many ports you need, but don't be surprised if it's several thousand dollars. Depending on how your rooms are arranged, you can do the same thing with a switch, and run ethernet cable to RJ45 keystones in each nearby room. This increases the amount of installation work you have to do (rig up ethernet and RJ45 keystones), but decreases the number of DSL connections you need to support. Be sure to buy a few spare VDSL modems to swap in for the ones which inevitably break over the years.

For the occasional WiFi hotspot, you can do the same thing, except the VDSL modem should be installed in a wall or closet in the guest room, and go to a switch. One ethernet cable goes from the switch to a RJ45 drop in a guest room, another to a WiFi access point either in the ceiling or some other place customers can't access.

First question is: is connecting one router to another (input internet into WAN port, output from LAN to the next router) possible if we talk about have a line of 6-7 routers on a total length of 100m of ethernet (on each side)?

Click to expand...

Cascading routers like that is generally a bad idea, as it ends up complicating diagnosis and troubleshooting if you experience network problems. You cannot troubleshoot the network by sitting at your desk. You have to take a laptop out to each router and plug it in manually.

Second: Long story again, we have an input of internet of actually 3 cables (3 devices on the roof to get the signal from the other side of the river), but our current gateway router has only 2 WAN ports. Is there a solution to have all three combined together?

Click to expand...

That's called bonding. Talk to your ISP about it, as it needs equipment on their end (or on a VPN somewhere on the Internet) to get it working. You basically construct a tunnel between your main router and a router at your ISP or the VPN. The tunnel then transmits traffic over the 3 Internet connections. A server or specialized bonding equipment does the work of converting to/from each router's network connection to the tunnel running over the multiple bonded channels. I believe Mushroom Networks is the company that makes most of the bonding equipment.

Routers with 2 WAN ports generally can only use them as fallback (keep WAN2 as a spare and use it only if WAN1 goes down), or for separate traffic (a single download can only go at the speed of WAN1 or WAN2). Bonding combines all your network connections into a single WAN port (a single download can go at the speed of WAN1 + WAN2 + WAN3 + ... + WANn.)