Ste - Quote the following at them!!
Sale of Goods Act 1979.
Supply of Goods and Services Act 1982.
Sale and Supply of Goods Act 1994.
The Sale and Supply of Goods to Consumers Regulations 2002.
They appear, at first glance, to have breached their contract with you.
Wherever goods are bought they must "conform to contract". This means they must be as described, fit for purpose and of satisfactory quality (i.e. not inherently faulty at the time of sale). Depending how long you have had it, it doesn't sound like the modem is "fit for purpose" or of "satisfactory quality" if it's broken already!! Depending on how long you had the other 3 modems for, the same will apply to them!
Goods are of satisfactory quality if they reach the standard that a reasonable person would regard as satisfactory, taking into account the price and any description. This appears not to be the case with you as it has already broken!!
(Aspects of quality include fitness for purpose, freedom from minor defects, appearance and finish, durability and safety).
It is the seller, not the manufacturer, who is responsible if goods do not conform to contract. If goods do not conform to contract at the time of sale, purchasers can request their money back "within a reasonable time". (This is not defined and will depend on circumstances).
For up to six years after purchase (five years from discovery in Scotland) purchasers can demand damages (which a court would equate to the cost of a repair or replacement).
A purchaser who is a consumer, i.e. is not buying in the course of a business, can alternatively request a repair or replacement. If repair and replacement are not possible or too costly, then the consumer can seek a partial refund, if they have had some benefit from the good, or a full refund if the fault/s have meant they have enjoyed no benefit.
In general, the onus is on all purchasers to prove the goods did not conform to contract (e.g. was inherently faulty) and should have reasonably lasted until this point in time (i.e. perishable goods do not last for six years).
If a consumer chooses to request a repair or replacement, then for the first six months after purchase it will be for the retailer to prove the goods did conform to contract (e.g. were not inherently faulty), the consumer need not produce any evidence that a product was inherently faulty at the time of sale.
If you need any more help, give me a shout mate
P.S. Thanks for the Contract Law Revision question