So that room is better. I wonder if there could be a power line cabling issue here.
Have you done a reset on these units? 
You wouldn't need to take the router, just move the 306s to a different house of possible. 
Do you have permission and access to these units? 
There's not additional FW support for these units so possible they could be bad if you can maybe test these in a different house hold. 
If not, I would recommend that you phone contact D-Link support and get additional help and review on this:
Technical Support can be reached at: 1-800-361-5265 
Consumer/Retail Support 
English: 24 hours a day, 7 days a week 
French: 12:00pm-9:00pm EST, Monday-Friday 
Business/Enterprise Support 
English: 9:00am-8:00pm EST, Monday-Friday 
French: 9:00am-5:30pm EST, Monday-Friday 
Customer Service 
English: 9:00am-8:00pm EST, Monday-Friday 
French: 9:00am-5:30pm EST, Monday-Friday