How does the Link Amp mod affect the maximum link range for portals in Ingress

You have to go there and hack the (destination) portal to obtain the key (yes, you need to hack "friendly" portals anyway to get supplies), or trade keys with team members by dropping/acquiring them.

Also, resonators "drop" keys which are used for links when one end is blown up that you can pick up.

Turned out that the inability to deploy the 8th resonator was a bug - I wish I'd taken a screenshot. When I got back to my computer, I looked and 8 had been deployed. It must have been I kept tapping deploy on my phone faster than something could handle it, and it just didn't show up. I'm wondering if restarting the ingress app on my phone would have fixed the issue as well.

As far as being unable to link it could have been for a number or reasons that I didn't know at the time. Portals link distance is determined by the level of it's resonators, and I may not have had sufficient resonators.

Level   Range
1       160m
2       2.560km
3       12.96km
4       40.96km
5       100km
6       207.36km
7       384.16km
8       655.36km

It also could have been because the link I was trying to create crossed the link between some other portals. It wasn't obvious to me that this wasn't possible at first, but makes sense and you get a message telling you why you can't link to a portal if you click confirm even though it's greyed out.

Finally, it could have been because portals only allow a certain number of links. You can only have 8 outgoing links and 8 incoming links. When this happens you'll see something like "Origin Link Capacity Reached" when you try to confirm a link to an unlinkable portal.