@StefanA 

Update:

I can cast to all the Cromecasts and Android units when I connect my laptops to the network via an Ethernet cable. 

But I can not cast when my laptops are connected via Wifi. So there must be something wrong with the Wifi-network. 

I have two suggestions of possible errors:

1. There is an error in the configuration of Wifi in the router. The config is standard all the way including that AP isolation is NOT enabled.

2. There is something wrong of the software for Wifi of the router. Maybe an error in DNS for Wifi-software? It seems that this error is common in various TP-Link routers. 

Any suggestions?

/StefanA

@StefanA 

SOLVED - (probably this is useful information for many users of TP-Link routers). 

The problem was on Windows 10 and Google Chrome. I have the latest official version 87.0.4280.141 installed. I downloaded developers latest version of Chrome Canary (latest official version 89.0.4386.1) and now I can cast to all Chromecast and Android units. I hope Canary is stable enough for daily use. Future will tell. Hopefully Google soon will release the latest version of Canary as an official version of Chrome. 

The official version of Edge has the same problem with cast not working and I have not tested the latest version of developer product. I leave that mission to someone else. 

/StefanA