GTM Best Practice Design Question

I would like advise on what the best approach would be for the following scenario I'm facing right now

We have 2 Data Centers

1. HQ (1 internet link ONLY! no possibilities to get another one )

2. DR (1 internet link ONLY! no possibilities to get another one )

HQ has 2 LTMs 3900 HA

DR has 1 LTM 3900 standalone

Phase 1 up to here up and running!!

Phase 2

1 GTM on each site for GSLB

Scenario:

Customer would like to provide Global Availability to his internal applications and also his E-commerce site but public/external DNS resolution for his website is hosted on a 3rd party provider (so far no problem... we will tell the provider to use CNAMES to redirect WIDE IP traffic to GTM)

Our problem is in the event Internet Link (router) goes down at HQ site and traffic coming from the provider can't get to GTM ? (GTM will have a listener on the same subnet of the internet link)

Since 3rd party Provider will still hand-out GTM's public address for resolution

HOW can the provider can tell when there is and Outage at customer HQ site?

(so his DNS servers can respond with GTM's public address at DR site? )

I know that through standard DNS round Robin provider can hand out both addresses (listeners) at HQ and DR sites respectively but is there a better way?

(I already mention including a 3rd GTM at tje provider and the answer was :is not going to happen this year)

I hope I made my self clear on what our Design issue is right now and any advise will be highly appreciated

PV

glenn