strange problem with an ISP and arris modem. Any thoughts ?

This is a little strange so I will try to explain the problem as clear as I could,

we got 2 ISPs in our location,

1 is Fiber to the node then comes is with coax and provides an Arris modem also we get public IP via DHCP.

ISP 2 FTTH and the thing I do not really like is they use PPPoE.

for the problem, we mainly have ISP 1 at the moment and the issue is when the Arris is put to bridge mode, download torrents, for example, Linux ISO kills the speed that the torrent speed goes to 0 and trying a speed test it also stays between 1 to 2 mbps, I also tried plugging the PC direct to the modem with sane problem I tried fq_codel with 3 routers with the same problem, now for the strange part putting the Arris to router mode this does not happen and I can't really understand why torrents download fine.

To make sure nothing is wrong with our hardware we tried the second ISP and everything works fine when is bridge mode, so something is going on with ISP 1 and really am curious what's going on.

I would love to hear any thoughts from you experienced guys.

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Thanks alot.